Because wearing masks sucks, that's why. It's uncomfortable. It makes my ears look funny. It's gross after a few hours of smelling your own breath. It's completely defeating in the context of trying to have normal social interactions, run meetings, etc. It makes going to the gym just that much easier to skip. Never mind getting screamed at by a self-appointed hall monitor in the parking lot of a store when you're in a hurry and just forgot to grab your mask when you jumped out of the car.
In saying that, I'm not coming from a 'muh freedoms, Tucker-Carlson right-wing tribalism,' because for some valid points here and there, they get lost in hyperbole and well, fake news. There's tribalism from the other side as well, and I think some valid points about civic-duty, etc., get lost in the same kind of hyperbole with, well rules-for-me-and-not-for-thee type hypocrisy (cf.: this list).
It's not like wearing masks is so great and so cool that we would decide to do it all the time, pandemic or not -- it's a burden. It's a burden I agree we should be willing to bear so long as it's effective in combatting the pandemic, but I think we can be honest in saying that it sucks.
I also think it's safe to say most people agree that it sucks because many elected officials couched the vaccination push in explicit terms of "if we meet this vaccination level, you'll get to drop the masks and go back to living your normal life." While I'm sure their primary concern is the health and public safety of their constituents, you can bet a close second is polling data showing that even the compliant base is getting restless.
In my state, this same rhetoric and tight-coupling of reopening to vaccination levels was augmented with millions in vaccination sweepstakes prizes, only to have that on the way to being countermanded weeks after meeting those goals, and well, it starts to feel like playing monopoly with the kid that changes the rules constantly (cf.: this. Again, I'm not arguing from a an anti-mask perspective here, I'm just answering your question.
I don't envy the people having to navigate and lead us through this mess, but heaven help us if in a year we're on to the sixth booster shot for the second wave of the omicron variant during WFH 3.0.