Either you are new or play in very tight groups. I don't know.
I've been at it for over 25 years now. In addition to my regular groups, I've had the pleasure of running pickup games for hundreds of players on Roll20 (if my stats are accurate).
Player abuse has been common in every edition of D&D dating back to the earliest editions. The entire reason two-weapon fighting has been reduced in effectiveness to where it is now is because it was abused in the earliest editions. The abuse complaints about casters has been there since 2E and reached its apex in 3E. The section you keep citing on goals of play, "play for group fun", and every other iteration of that idea has been talked about in every edition for three decades. Doesn't matter. Many players, I have no idea what percentage, have fun trying to make the most powerful character they possibly can. They will push the DM to the limits even if other players don't like it. They won't rein it in unless the DM forces them to rein it in. Maybe you kick players like this from your group, but such players will find new groups. Thus they become another DM's problem. And thus the complaints on this thread.
During my time playing D&D and other RPGs, I have encountered players like the ones you describe. But every single one of them was a reasonable person when, as a player or DM, I spoke to them about the goals of play which, as you say, have been around for a very long time. Once the goals of the group are aligned, players can indeed create "the most powerful character they can" and
still make choices that help the group achieve the goals of play. I didn't need to "put my foot down." I didn't need more rules or rules clarifications. I just needed to assume the best of them and speak to them on a person-to-person level and seek their buy-in, then watch with pride as they rose to the occasion. I've seen the pixie trick in my game - once - and it was
awesome. It led to an incredibly fun scene and the introduction of a whole fey subplot in the campaign. After that point, the player never used it again because he made a decision that while it was a powerful gimmick, it wouldn't be fun for everyone and wouldn't lead to exciting, memorable stories to use it all the time. In my last campaign, two of the players decided that their characters had to die because that was what would make for the best story at that moment. We "won" D&D by the definition of winning in the Basic Rules, in part, because of their choices.
If the players and DM don't agree on what it means to "win" or "lose" D&D, then perhaps it is inevitable to see the abuse that so many people have mentioned in this thread and others. But it's spelled out right in the rules for all to see, discuss, and agree upon. Perhaps a discussion is in order for such groups rather than more rules or rules clarifications. It has worked for me and maybe it will work for others. And I hope it does - not having to be on guard for player abuse makes it a lot easier and more fun to run the game for me.