If you are talking about day-laborers and so on, I agree that is very common (it's the only reason I still have physical paper cash anymore).
That said, physical cash--and even Bitcoin--is theoretically traceable unless you do a lot of things that are a pain in the ass--and none of the people I've ever dealt with have indicated doing any such thing.
But here's the thing: in the US at least, they don't tend to create some kind of massive IRS investigation to go after some laborer so they can recover a whole $2300 in back taxes or whatever. It's just not done. They go after people who evade hundreds of thousands or millions because anything else is a waste of their time. That's why this practice is done, not because these people had Monero.
I know people that only recently stopped using PayPal for payments only because it automatically takes out taxes--but they just moved to some other app. "Theoretically" they could be made to pay back taxes, but only if the IRS specifically targeted them which is unlikely.
Physical cash is theoretically traceable and yet this is how this kind business has been done in the USA for years and it works just fine. You don't need to use the same payment method that Bin Laden uses in order to evade a few thousand a year in taxes.
Of course even if people like this could only use a secure crypto like Monero for some reason, that market would be very small. You're talking about a fraction of the wages of a fraction of the population--and the lowest paid part of the population as well.