How weird. Must be a generational thing. During the TSR era house rules and home brewing was expected. Too bad the DIY aspect of the hobby isn't as much of a thing anymore. Or is it some weird appeal to authority thing where only official rules are good? There's a lot of great DM's Guild stuff and 3PP putting out great content. I guess they're not regular players and DMs like the rest of us. Their stuff is somehow special because it's in a PDF. Such a weird thing to turn one's nose up at considering the history of the hobby.
I think that newer players and DMs like having the rules spelled out for them, and given the larger number of new players in the hobby, they just don't have the experience of doing DIY.
D&D also evolved out of wargaming clubs, which in turn evolved out of military modeling. That entire hobby started out as DIY. You could buy model kits, but toy soldier painters often sculpted and cast their own figure molds. When the industry took off in the late 50s, a few mini manufacturers catered to the growing hobby, but those guys were all DIY enthusiasts turned entrepreneurs. Wargame clubs played the games, and all contributed figures so they could fight the battles, so groups would often build their own terrain, have club dues to buy materials, etc.
The first D&D game I played in as a kid, was at a D&D club that met once a week renting out an old-country buffet banquet room. They had 60 members, with club officers, dues, etc. The club "sergeant at arms" paired me with an older teenager to be my game chaperone, and we played in a very organized, old-school game (8 players, 6+ Hirelings, party caller, etc) through an old Judges Guild adventure.
Today's gamers grew up with videogames and casual gaming, without a commitment to another time-consuming hobby, so they just want to pick up and play, rather than spend hours prepping the same way a guy making a model ship, or painting 100s of Napoleonic figurines would. Plus, they don't belong to clubs of 10-50 players, who encourage prepping as part of the club mission statement.