That isn't the case at all. Balance is currently pretty good with a couple of exceptions and every patch a new build comes out - with any build you can clear the entire journey. Some builds are anyway extremely powerful. Current Tal Rasha/Manald Heal for example; though the issue is more about Archon being insane.
The whole issue resides in what the game has to offer. Since GRs are the only endgame, it's just natural that peoiple gravitate towards the builds that can make you go further; also worth noting that there's place for only one #1 build or 4man meta.
Even if a build is just 0.01% better, people will just focus on that because some youtuber has made a "NEW META GR703928572629586+" video guide.
If the issue is "i cannot find groups because people want all a Wizard", that's a players problem. It's players locking out other players because they don't have a meta build.
But WHY this happens? Because the only content is an infinitely scaling randomized dungeon, so sooner or later you have to swap builds/character, or stop playing entirely.
If there was other kind of content that has a fixed difficulty (example all on T13 level) you'll see many more builds being played. But this isn't the case.
Another example: look at PoE. There are meta builds that are extremely overpowered all over the place. Point is, that the endgame can be done with any build and the only difference is clearing maps faster or slower. And you definitely don't need it - there are no ladders tied to that; only the race to 99 (like D2) and labirynth clears (timed runs) which are both very specific activities that appeal only a portion of the playerbase.
TL;DR: you can do everything with any build and have no issues, because endgame has a very different structure.