Thanks, that's much clearer. You ask why people believe that God created man in His image on the 6th twenty-four hour day (well, actually you asked why He created man in 6 twenty-four hour days, but He didn't - He did it on Day 6). I can't answer for all people of course, just for myself. I believe it mainly because the bible says it. Over and over, we read, "A nd the evening and the morning were the .... day," which would make no sense if the days of creation were in fact many years. Nor would the commandment for a weekly day of rest make any sense if Creation days were many years long:
“9 Six days you shall labor and do all your work, 10 but the seventh day [is] the Sabbath of the LORD your God. [In it] you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who [is] within your gates. 11 For [in] six days the LORD made the heavens and the earth, the sea, and all that [is] in them, and rested the seventh day. Therefore the LORD blessed the Sabbath day and hallowed it.” (Ex 20:9-11 NKJV)