It is not so much a problem as a simple reality that the cost of fulfilling every reservation exactly is (for example) 10x then it is worth fulfilling only 99% of reservations at a cost of 1x.
Car rental customers are not going to pay 10x to get that extra 1% of guarantee, and you simply will not see that type of car rental business exist. It simply is not economical to build enough redundancy for the 1% of the time there is a problem.
Of course, it also is not economical to build enough redundancy for multi year pandemic situations.
I’m just making assumptions, but the point is a sufficiently small number of people are being disappointed such that the business is still viable (shown by the fact that it stays in business even though they cannot guarantee every reservation).
Presumably the market is capable of coming close to the solution where people are willing to pay $x for a y% chance of having their needs meet, such that sufficient people will not pay $x+z to get a 100% chance of having their needs met.
Could the business not be 'viable' owing to the fact that there is simply no alternative? If the choice is between making a "reservation" that may or may not be available and between definitely not having a car, obviously everyone is going to pick the former, but that could be within bounds of 99% of those picking the former still being disappointed, just having no other option.
There have been many car rental companies for many decades competing with each other. If it were true that sufficient people were willing to pay for 100% reliability, then it stands to reason one of the car rental companies would have championed that guarantee and succeeded.
It is not like it was an unknown issue, having even been in the plot of one of the episodes of one of the most popular television show of the 90s.
Even today, there are 3 car rental companies, Enterprise, Avis, and Hertz, and any one has the option of offering a 100% guarantee, but they do not.
Hotels do not either. And neither do airlines. You can get walked to another hotel, or bumped to another flight (assuming you are not a VIP of some kind).
Which tells me guaranteeing the last 1% (or whatever small percentage of people that cannot be accommodated) is too costly and customers would rather save money instead of pay for the redundancies needed.
It has been a problem for a very long time.