The rule changes is a big one. So many tables do things like "max hp for all PCS!", and worse have the "no PCs will randomly die or be killed ever". Optimizer Jerks can thrive in such a game
I mean, non-optimizers benefit massively more from both of those changes than optimizers lol.
Optimzers will usually have PCs with higher HP and ones which are less likely to die anyway. If a DM does something like that, if optimizers make say, a 30% gain because they can maybe consider having a lower CON if that benefits them, non-optimizers make like a 100% gain, because they can just do whatever the heck they like and know the DM won't kill them. They don't even have to worry about it.
But it gets much worse when you have the DM that just sort of has foes sort of attack for a fun combat romp where the PCs will always win with zero chance of PC death ever.
"PCs always 'win' combats" describes easily 80-90% of campaigns in
traditional RPGs (PtbA/BitD and other narrative stuff is a bit different, so we'll exclude it) run by pretty much everyone outside of the horror and OSR/NSR genres because in
most RPGs, particularly D&D-derived ones, the only possible results of a combat are:
A) PCs 'win' (whether at great cost or little)
or
B) TPK (which the DM may mitigate by saying "actually you were all captured" or the like, but is still a TPK).
This is especially true if for "always" we mean "99% or more of the time" rather than a true 100%.
The trouble with so many RPGs is that due to their round, initiative, action, and movement structures, a "fighting retreat" or similar disengagement simply isn't possible RAW/RAI, and there's no possibility most enemies will ask for or accept quarter/surrender/disengagement. Best case tends to be an exceptionally fast PC or two might be able to do the old "you don't have to run faster than the bear..." to the rest of the party.
We can pretend that isn't the case, because all actual plays across the entire internet show it is (again, outside of horror and OSR/NSR) at least 80-90%.
So unless that's taking place in a horror or OSR/NSR context, that's not really a valid complaint at all. You're just going back to what I said earlier - you're complaining about your own bad choices - you participating in games you inherently dislike the tone of and/or wishing to change them so they play different genre.
You're essentially saying that, say, 25m of the estimated 30m D&D players are "doing it wrong" lol. Which is bold. It's nothing to do with the "weak DM" point either - that's separate. This is just a playstyle choice, which might have a strong or weak DM behind it.