Supposedly the US rental companies assumed their business would tank due to lockdowns so sold off much of their vehicle inventory. Then when people turned out to travel anyway through the pandemic, they tried to buy more vehicles and found they were hard to get due to supply chain issues.

I think it depend on the specific location. Here in Bozeman the airlines are complaining they can't fill planes because travelers can't rent cars here so they travel somewhere else or stay home. But for example LAX seems to have cars, albeit higher price than before.