There are loads of reasons governments can't afford up-front capital expenditure! They have an income stream (taxes) but that all goes out the door on existing liabilities, including financing existing debts. They can only get money by raising taxes or cutting existing services. The third way is to try and attract external capital, e.g. "attracting inward investment" or "public-private partnerships", where someone else puts the money down, the trains/hospitals/schools/etc. get built, but the investors who paid for the things own the things and the government pays to use them for decades to come.

This annoys the hell out of future governments, because it's effectively tying their hands, but it also lets the current government look like it's doing a lot more in its short term without raising taxes.