In a monarchy, long term interests win out over short term interests because they're dynastic and stay within a family. There is a natural incentive for long term thinking that comes with a monarchy.
This problem is not democracy itself, but the incrementalism that occurs in democratic systems. At least in representative democracies, every few years another person is elected to carve out provisions to benefit their special interests. In theory, this would be the constituents. But in practice, after the politicians realize voters are unorganized, uninformed, and as self-interested as everyone else, the politician also focus on themselves (and you would too). Compound that over hundreds of years and it's should be clear as day why democratic governments inevitably grind to halt, become ineffective, collapse, and are recreated from a clean slate.