The fascination between young boys and dinosaurs must be studied. There's something imprinted in the DNA for it to be so common lol.
Essentially, as hunter-gatherers every group of humans needed one or more experts on the local megafauna, so virtually all of us have some degree of fascination with big animals baked in, but some have more than others. I've also heard the same hypothesis used to express young boys' obsessions with trucks and trains.
Probably a developmental phase somehow connected to quadrupeds. Large object of interest. Four supporting thingies.
(Girls surely do too, but I cannot attest to that)