Any risk analyst could tell you all the factors you don't take into account when you do a simple comparison of renting a new, cared-for, insured vehicle for which someone else is responsible vs. the crapshoot of buying something second-hand, which you now have to take on, whatever goes wrong.

I also doubt you are taking fuel into account with this comparison, which was the primary source of savings mentioned when you own a small car and rent a truck as needed.