Time dilation is indeed an experimental fact.
So as velocity increases decay rates slow. But if you use today's slower rate to calculate into the past without taking into account the decay rates increase as you go backwards, you would come to the incorrect conclusion that the earth was billions of years old.
Time dilation does exist, but what you apparently still fail to grasp is that it is
relative. The rate of time passing in Earth's proper frame never changes, whatever our movement. What changes is the rate of time we measure in frames moving relative to us, and the difference in the rate of time we measure for such frames depends on their relative motion with respect to us. Likewise, from the point of view of other frames, their own proper time never changes and it's our rate of time which they measure that changes; if we measure their time running slow, they'll measure our time running slow.
Since there's no absolute frame of reference, with respect to which frame are you suggesting that the rate of passing of time on Earth has changed, and how do you propose it makes any difference to the age we measure for events in Earth's proper frame?
By products of this is that mass also increases due to an increase in energy, hence we no longer have dinosaur sized animals because they can not exist in today's higher mass.
Mass increase, like time, and length contraction, is also what we measure for other frames that are in motion with respect to us, i.e. it is
relative. As far as we are concerned, it never changes in our proper frame. Frames in motion relative to us will measure a change in our mass, and rate of time passing, and a contraction in the path of our relative motion, and we will measure corresponding changes in theirs, but from the POV of each frame their own mass, length, & time never change.
So again, with respect to which frame are you suggesting our mass has increased? (hint: it would have to be moving at a significant fraction of the speed of light relative to us to measure the kind of changes you suggest).
There have been ill-informed suggestions that Earth's gravity was weaker 65 million years ago, but a moment's thought will tell you that any difference significant enough to allow creatures to grow several times larger than otherwise, would disturb Earth's orbit, that of the moon around the Earth, and change the sun's size and rate of fusion; there's no evidence for any of this.
And if you're suggesting it's because of the big bang, it would also apply to the rest of the solar system and the whole galaxy too. We don't see any such disturbances and we don't see the mass of stars & planets decreasing with distance from us (i.e. back in time), or any other consequences that would result.