Absolutely no one is saying this. This is a huge straw many that you are creating.
What is being said is that if you absolutely refuse to try other games, despite gaming for years on end, that you have a limited band of experience compared to someone who has at least tried other systems from time to time over the same span of years. And that it shows in these discussions quite clearly because you see people pontificating about other systems that they've obviously never played, trying to "prove" how those other systems are inferior.
Good grief, mention a story game in a thread and it's like chumming the water. Suggest using a technique from a more narrative based game than D&D and people lose their poop, despite having never actually had any direct experience and those same people will argue until the cows come home that the experience of people who do have direct experience with those systems is wrong and they just don't understand the games that they play.
The problem is, we ALL play D&D. Some version thereof anyway. We're on En World because we play D&D. Most of us have been gaming D&D for years. We all have considerable experience with the systems of D&D. The difference is, there is a rather loud cohort of D&D players who absolutely refuse to entertain any notion that there might be other ways of doing things that might actually work better.
And it's because there is a rather loud cohort of players for whom D&D is the be all and end all of TTRPG's and no one could possibly have an idea that isn't done better in D&D.
I don't care what other games you play or how often. Play a variety of games multiple times a week, once a month a few times a year? Go for it. The fact that you are trying to create a a strawman out of nothing is telling. It's condescending for you to tell me I would be better off if I played more games. If anyone on these forums is saying that D&D is the be all end all for anyone other than themselves please provide a quote because I haven't seen it.
I have never driven a semi, my experience with motorcycle is limited to approximately 20 minutes, have never once tried to fly a plane or use a unicycle. I may not fully understand all of the problems or benefits people face with any of those, and I may well have some misconceptions of how they work. If I try to tell someone else who does use them how they should be doing it, that's my bad. Meanwhile I would not be a better driver of a car after driving a semi. Unlike you I'm not telling you what games you should or should not play.
You don't know me. You don't know how good a player or DM I am. So stop