Building towers is capital intensive, yet they generate tons of income and have a pretty good resale value.
Typically you start with a small amount of money, and killing each wave generates enough extra money to build more towers, and just barely kill subsequent waves.
There is usually an option to make waves come faster, which generates more money.
If there were ever any real-world tower defense game, people would naturally organize banks to lend people money, so that they could build all their towers at first, and speed up the first 50% of waves to generate profit. As long as the interest is less than the amount of extra money generated by making the waves come faster, banks & players would both profit.
Another never-considered aspect is that most levels end with all towers in place on the board. This is a huge waste; if tower resale value is 50% of build cost, then the actual amount of money "lost" on capital investments is only 50% of total money spent! i.e. if a level's enemies are worth a total of 5000 gold, then at the end of a normal level you should just barely kill them all, but still have all your towers, value 2500. Any loan you received at the start would have a built-in return amount of at least 50%.
Creating these banks wouldn't even take government action - it would be such a gain to just lend someone a few extra gold at the start of a level - people would do it naturally.