What year did you start? Do you think being exclusively d20 that long is a common experience?
There are outliers, obviously, but I think spending years, possibly even decades safely ensconced in the d20 / D&D cocoon is pretty normal.
I played BECMI / Rules Cyclopedia as a kid in the late '80s through early teen-ager-hood in the mid-'90s. Then took a long break, then picked up playing again in 2003 with a friend who was way into D&D 3.5. Played 3.5 from 2003 through 2009, then played Pathfinder 1e from 2009 through 2011, with some (very) short stints into Monte Cook's Arcana Evolved, D20 Modern, and Star Wars Saga Edition.
I did own Top Secret S.I. as a kid as well, though I never actually played the system. It's a percentile roll-under. I didn't know it at the time, but it's pretty clearly a spin on BRP / Runequest. Without access to the internet at the time (1990 was a different world), there was pretty much no way to bounce ideas off people on using it, though even at a tender age I converted a magic system using percentile skill rolls to tack on to it.
But even that tiny exposure to an alternative system was basically an afterthought. By the time I actually
played anything other than a d20 system (GURPS in late 2011 and Savage Worlds in early 2012), It had been what, more than 20 years since I'd even looked at something other than D&D / d20?
At that point, if I had had any inclination for homebrewing rules, it would have been solely from the perspective of trying to "Fix D&D / d20" from within its known structure.
For the "old school" among us, the outliers in most cases are individuals that started with something other than D&D. They arrived into the hobby via Vampire: the Masquerade, GURPS, Runequest, something Palladium, Champions/HERO, or Rolemaster. If you were introduced to RPGs in the '90s, I'd say there's a 60% chance you entered through D&D, a 35% chance through VtM, and 5% chance for the others. But even in the '90s, entering the hobby through VtM would still be 1.5x less probable than through D&D, and anything other than D&D + VtM were extreme outliers.
I think it's safe to say that a vast majority of "current gen" roleplayers arrived through 5e (with an enormous helping of Critical Role) almost exclusively.