That's due to a miscalculation around the sudden death of demand due to a global pandemic (which bankrupted a bunch of car rental companies, including Hertz). Now that demand is back, they're rebuilding their inventory, as exemplified by this 100k car order. Things will stabilize, and lots will mostly go back to having plenty of inventory in not too long.
Not the person you asked the question to, but... I've rented recently (1 month ago). The parking garage was filled with an absurd number of unused cars. There were entire levels of the parking garage which were filled with cars which weren't being used because the rental car companies associated with that floor weren't even open. Meanwhile the rental car operator which was open wasn't even close to having rented out all their cars.
Their lots really aren't "always full of cars". A lot of rental companies, especially franchises, have sold a lot of their cars due to reduced demand and insane used car prices.