I've flown quite a bit in the past year - while there's been more cancellations than pre-pandemic, the last few weeks are outliers.
"Capitalism" can't produce new staff instantly at any price when staff are in industries that require substantial training/testing prior to being allowed to work after hiring (or after any long furlough/hiatus of not working, even if they remained officially employed).
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Anyway, you've got a perfect storm of issues.
Airlines have depleted crews from cutting too deeply in the early pandemic. Training/retraining large numbers of people takes lots of time, only so many people are actually qualified to teach or test them.
Airlines were already running what staff they have with as minimal spare capacity as they could get away with to meet demand that's come roaring back far faster than expected, especially for the holiday season. The US is currently running ~83% of 2019 passenger loads (12/20-1/4 passenger count in 2019/20 - 36.6m, 2021/22 - 30.7m), per the TSA checkpoint numbers.
And then....you have a new COVID wave that's leading to substantial portions of the workforce coming down with it and being unable to work for at least a week during their absolute highest load period.
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Are the airlines entirely "not at fault"? No. They cut too deeply in the early pandemic. And they've been too greedy chasing $$$ for renewed demand and added too many flights too quickly, cutting their spare capacity for issues that could crop up to nothing...and now they have.
But the severity of this COVID wave is significantly worse (in terms of # of people infected at once, not referring to illness severity) and pretty much every area of the workforce (that's actually in-person) is having similar issues. Many public schools in the US are basically just functioning as babysitters this week if they're open because they have too many teachers and students out to actually run classes normally.