Why ask how it could happen...there has been a shortage of cars going back to the beginning of the pandemic as auto manufacturers trimmed back their production of new cars - drastically. The US car rental industry has always been highly dependent on the overflow of OEM vehicles for decades.
During the pandemic and with the drastic curtailment of travel, most car rental businesses sold down their fleet inventory to what became a robust used car market. Now, as we return to leisure travel season, don't be surprised if that full-size car you were looking for has over 40,000 miles on it and is over $100 per day.
It will take quite some time to go back to the old days of low rates and fresh cars. Hopefully, we all will at least enjoy complaining about it as we will at least be back traveling once again!
A rental-car shortage is pummeling the US and may only get worse as travel increases.