Why?
Lots and lots of players will put up with TONS of bad DM's. The hobby is replete with stories about them. Some put up with it because they just don't know any better. Some put up with it because they want to hang out with their friends. Heck, of the times I've seen player revolts, it wasn't like it happened at the first session. Player revolts, IME, occur after weeks or even months of play.
That about 1 in 4 DM's is bad should not shock anyone. 1 in 4 of pretty much anything is bad. They might be bad due to inexperience. They might be bad because they learned to play from other bad DM's. They might be bad for a thousand reasons. And that's not counting the thousand more reasons why someone might just not like a particular DM. Heck, I've had lots and lots of players sit at my table and not stay very long. Just mismatches in playstyle or whatnot. Happens all the time.
But, honestly? I've sat at some truly spectacularly bad tables. The DM, when questioned if he thought his NPC's were more important to the campaign than the players very honestly said, "Of course". :wow: The DM who was so cack handed railroady that the entire group quit on the spot. The DM whose personal grooming was so bad that the group simply couldn't sit at the table anymore.
Am I a great DM? Nope. I'm average at best. I try my best, and I hope my players enjoy my game, but, at no point am I so arrogant as to think that I'm fantastic at what I do. I'm lucky in that I've managed to build groups of like minded players over the years and there have been some spectacularly good groups that I've been lucky enough to be a part of. But a group is always only as good as the members of that group.
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Edit to add.
Let's not forget that the overwhelming majority of players out there only play for a year or two. There is a constant cycling of players going on. Of the gamers I gamed with when I was in uni, I'm the only one that's still playing. Sure, my current group has been gaming for a while, but, that's not normal. How many gamers go to a year of organized play and then never play again? How many DM's get the Beginner Box for Christmas, play for a while and then move on? Heck, the old Moldvay Basic set sold something like a MILLION copies, yet the gaming population was never more than a few hundred thousand after the bubble burst. Until the recent bubble with 5e, the gaming population was tiny. If people stayed with the game, the game would have grown huge long ago. But they don't. My experiences, your experiences and everyone else's is such a tiny slice of the fandom. Why is it so hard to believe that there are a minority (say about 1 in 4 or 1 in 5) DM's out there that are bad?