[pmyteh]

It's mostly because the ability of the economy to produce positive economic (but not direct financial) returns on very cheap credit is basically infinite, and borrowing money to invest in all of these things would need a huge amount of extra tax revenue to pay off the loans, which is hard. (Even in principle it's not straightforward for government to capture the consumer surplus of infrastructure investment. And in practice tax increases are politically problematic). So it's rationed, instead.