You misunderstand my position. They aren't helping people out of altruism. I never said they were. But their presence gives low skill workers, many of whom don't have the aptitude or ambition to work in the trades, a place to work. Surely you don't think that the lady who is 50 pounds overweight is going to climb ladders to run conduit or crawl under houses to unclog drains. But she can work a register, stock shelves, etc. The presence of a Walmart store gives those people a place to work, and also keeps prices lower versus the local businesses they replace (whether that's good or bad for a community is an entirely different debate).
I can't speak for the entire country, but where I live and in the geographical footprint my business serves, there are plenty of affordable and accessible trade schools. Every plumbing, HVAC, or electrical company is short handed. Not everyone has the desire to maximize their earning potential.