Because it's ridiculous. He's to blame, she's still a dickhead.
But you've got someone here literally saying that it doesn't matter that he's shittier! It does! It matters more than anything else! It's the key and crucial point!
When you say "it doesn't matter that he's shittier, she's still a shit", you are focusing emphasis on the OW - i.e., blaming the woman for the man's indiscretion. You (generic you) are still trying to find ways to make sure the woman gets a disproportionate share of the blame, even though the only person responsible for a personal pledge is clearly the person who made it! You think that as long as you give a bit of lip service to him, you can still make the point you really want to make - that SHE'S A SLUT!
It's 2018 and we are STILL DOING THIS!
I'm not trivialising the pain and devastation of affairs, not for one second. But it isn't respectful, or in any way helpful, to that pain and devastation to misdirect blame for it. It doesn't help people to make decisions if they're still trying to find ways of focusing on blaming the woman, or at least the person who's not in the marriage. It doesn't help the healing process. It doesn't help the shitty misogyny of society which, as we've seen upthread, can see both wives and OW blamed when a man starts tomcatting around. We think we don't do this any more, we do it all the frigging time.
You can hate the OW, sure. You can think she's a complete bitch, chances are you're right. But you simply cannot make her responsible for your husband's vows. He made a pledge to you, so when he breaks it, focus on HIM.
You just cannot make everyone the guardians of other people's marriages.
Nobody has yet answered my question about the husband who propositions 50 women and remains faithful only because they all say no. Is that marriage OK?