Probably because the wages are very low and the work loads way too high. Some link: "How the current worker shortage is really a wage shortage" https://www.nynmedia.com/content/opinion-how-current-worker-...

"It’s not a labor shortage — it’s a wage and workers rights shortage" https://thehill.com/opinion/finance/570441-its-not-a-labor-s...

"Our employment system has failed low-wage workers. How can we rebuild?" https://www.brookings.edu/blog/up-front/2020/04/28/our-emplo...