Did they just not want other generations to have nice things?
Same reason we're messing up the economy now. This generation is making all the mistakes the baby bombers made and worse.
We've been even more short sighted. Instead of reversing unsustainable policies we've doubled down on them.
For most of human history, life was simple and we only had to plan a few years ahead. We're not really wired to see long term consequences. Even if we do see them, we're not really wired to care.