I... maybe think the same thing about the tradespeople shortage as I do about the nursing/teaching shortage? We super need them, wages are going up, but supply isn't, which breaks the law of supply and demand (more or less). I think the cause here is a culture one, like there's not a lot of prestige or money in this blue/pink collar kind of work (generally, not always), so people (millennials) went into white collar professions instead, so we have a glut of people with bachelor's degrees and college debt, and a shortage of people with trades credentials. I'm handwaving a lot, but it seems intuitively true to me.
I do think that it's a tough field to keep "staffed up" since it can wear people out pretty quickly, so it needs a constant influx of newcomers.
I think you're probably right about attrition, also the work can be hard on people. But also I think I read somewhere there just aren't a lot of people going into the field. Something about parents encouraging their kids to go to college and become white collar professionals instead of blue collar laborers. That rang true to me too.
I think a lot of this probably boils down to our insistence that cars be huge parts of our lives? It's really incompatible with the 15-minute city just because of the space requirements (parking at destinations, parking at homes, street width), and let's not forget maintenance costs and noise/pollution externalities. But I guess in the face of no public transit, what are we supposed to do?