Because the Father had forsaken Him; and so had the Holy Ghost.
Jesus didn't just "say that for our benefit" that was a real experience He endured.
Part of what the atonement entailed was to bear wrath for sin. Well to bear wrath for sin means to be forsaken by God. Those that are cast into the lake of fire at the end of time are cast outside of God's presence. And if you notice, they bear that punishment as whole individuals (both body and soul). This is why in the end all humanity is resurrected; not just those who receive eternal life.
So the majority of the "3 days and 3 nights" in the heart of the earth, Jesus endured as a whole person (body and soul). His soul was severed from His personhood when the angel of death came at the passover and consigned His soul to hades. If He didn't have a Divine nature, He would have died then and there.
Now if you go read psalm 139; (I'll give you a link to it in another study I did) you'll get a fascinating explanation of Jesus's experience of being human and how what He came to understand about Himself (His body), how He was "created", why He was "created"; interacted upon His psyche in the relationship He had with the Father and Spirit.
It's a fascinating psalm to research in the Hebrew. Jesus was very "specially made" and He knew that. He was unlike any other human being that ever existed and He grew to learn the full implication of what that meant over the course of His life. He knew at 12 years old that He was the Messiah. Now did He know that meant these leaders He was speaking to in the temple (at 12 years old) were going to crucify Him? At that time; probably not. Did He know He was the sacrifice? Probably, because of the story of Abraham and Issac.
Now this story about Jesus in the temple at 12 years old is actually rather comical, especially if you know anything about human developmental stages. LOL
So here's 12 year old Jesus thinking "I must be about my Father's business." So I'll go to the temple; (which was probably the first time He actually remembered being there, because 12 years old would have been the first time he was "required" by law and custom to go and "perform his religious duty" as a male Israelite.)
So here enters another aspect of this story that we can ascertain both by Scripture as well as anthropology. At 12 years old, Jesus was no where close to even
starting puberty. Historically, He likely would have been between 16 and 18 years old before starting puberty. Puberty takes about 2.5 years to complete in humans. We know historically people entered puberty later than they did, starting in about the 20th century. (In 3rd world countries this is still true; puberty is delayed until the latter teen years.)
So here's little bitty Jesus; probably no bigger than 4.5 feet high and 70 or 80lbs soaking wet! The average Jewish male in the 1st Century didn't top 5ft. 3inches high or more than 130lbs. The average Roman soldier was 5ft 6inches high and 160lbs.
And developmentally, bitty Jesus here had another milestone He hadn't hit yet; mylenezation of the brain. The peripheral nervous system is mylenized first. That begins shortly after birth (which gives one the ability to move their body through space). Mylenezation of the brain starts at about 12 years old and takes until one is about 18 or 19 years old to complete. Mylenezation of the brain (among many other things) gives one the ability to foresee and think through possible consequences to their actions. I.E. If I "get lost" Joseph is probably going to get mad. (And Joseph did get mad. That's implied in the Greek.)
So Jesus is skipping off to the temple thinking Oh well, what does Abba want me to do and so He's talking to scribes, pharisees and doctors of the law; and these guys are impressed with His understanding. At that point they probably wanted to take Him and stick Him in one of their yeshiva schools. (Which possibly / probably would have subjugated Him to sexual abuse; because that was common in those types of environments in the Hellenized Jewish ancient world. Take a look at Herod Antipas - the Herod who crucified Christ was probably homosexual.) Jesus also would have run the risk of getting abducted off the streets of Jerusalem and sold as a temple prostitute in these Roman / Greek temples. (That was also a common thing that happened to kids who were wandering about alone.) He probably wasn't particularly aware of those dangers; (or if He was; He didn't think about it - 12 year olds don't. That's part of the developmental phase they exist in.)
So 3 days later Mary and Joseph finally find Him and He says to Mary; "How is it you're looking for me? (or why would you be looking for me) Don't you know that I must be about my Father's business?" Then all the sudden everyone is looking at Him. (Well, Jesus that was the "wrong" thing to say.)
Then at some point DAD clues Him in: I appreciate your enthusiasm. You got spunk Kid; but You also got a lot to learn. Go back to Nazareth!
Jesus: Ehhh, OK.?!.....
(And Joseph is over there ready to paddle His little Divine behind.)
Scripture says Jospeh was a just man though; so he probably just swallowed his anger at this hapless adopted little child, walked over, literally picked Jesus up and walked out.
(Abba Father, this is embarrassing. I feel really small right now.)
LOL
Psalm 139 - last page of study.
What Did He REALLY look like? Chapter 4: Psalm 139, a bible fanfic | FanFiction
Yes, He did.
Psalm 16:10
Acts 2:27
1 Peter 3:18-20
So he can not deceive the nations. Revelation 20:3
Yes, these are differing schools of eschatological (end times study) thought. Dispensationalists don't believe the 1000 year reign has started yet. A-millennialists do. I'm an a-millennialist. I believe Satan was bound in the bottomless pit either just before the atonement started, or at the resurrection. the end of Ephesians 1 tells us Christ was given all reign, of domains and authority at the resurrection. Satan was cast out of heaven just prior to the atonement starting. That is recorded in John 12.
I believe we are now at the point where Satan has been loosed. And I'd be surprised if Christ doesn't return in our lifetime.