For me: It started with the Scarab Lord and Death Charger's reigns in Vanilla. Those were two mounts you saw and you turned to look at (Deathcharger on alliance for sure, not sure if horde had the same reaction).
I grinded out the Death Charger in Strat and got it first patch of BC (back when I could no life as a middle schooler, rip
). The whispers I got gave me a rush I hadn't felt before, and it solidified my mount collecting. BC was the worst for me, however. My buddy and I would duo ZG. He would get the tiger, I would get the raptor. Tiger dropped, he got it, raptor never dropped for me. The one time he ran without me, the Tiger dropped again, so I never got the tiger. In ZA, my guild kept having people leaving after they got their bear. Final run of the year I convinced people to come help me get it. Shaman forgot Fish Oil for the biggest skip (water walking across the lake to the last boss of the run). Missed it by one second.
Because of missing those mounts, my drive to collect them got higher than before. It also was around that time I realized how much I loved raiding, but you would always replace your gear. Mounts stayed with me forever. So because of the rush of people noticing cool mounts it got me into raiding, which got me more into collecting mounts than before because they will always be there.
One of the reasons I've quit playing for the most part is due to all the store mounts. It's really started to feel like the store mounts get all the cool flashy stuff, and most of the mounts in-game are pretty blah. I think they've done a little better recently, but it's a far cry from what it feels like it used to be. That could be my nostalgia goggles from my childhood, it could be that I have higher standards than before due to how intense some mounts are, or it could be that there's so many flashy store mounts they tend to grab your visual attention more than the ones you can earn in game, but whatever it is, it hurts my soul