Because that is what happens with every boom and bust. When energy prices crash --- you have mass layoffs, capex is slashed, and focus turns to survival. We've had a couple significant oil crashes in the past decade (2015, 2020) and three crashes over the past 15 years --- leading workers to say enough is enough and they've moved on to more stable and likely more enjoyable work that pays decently well. Could they make more money in the oil fields, most likely but not worth the hassle of the boom and busts and its real work. So, we stand today with a shortage of labor and equipment and there's not much anyone can do about it, unless borders are opened to let in people who are willing to get their hands dirty.
There's labor shortages in most industry today --- from airplane pilots to bus & truck drivers to oil & gas workers to health care workers to restaurant workers to professionals (accounting, finance, legal). COVID has killed a million+ Americans, but, it also provided families the opportunity to reassess their priorities resulting in the great retirement and more families being content with only one working parent.