The problem is that politicians aren't geared toward factoring the costs and benefits of long-term solutions. This institutional problem of incentives leads to a situation where politicians don't have to fulfill long-term promises--even if they're in power for > 2 or >4 years because...
(1) they fight over votes in the short-term (every 2 to 4 years)
(2) they can shift the blame of the long-term costs on others (politicians, bureaucrats, individuals within the market) while shifting credit to themselves for short-term gains to their interest groups (donators/contributors) and select groups of constituents (e.g. union workers, welfare recipients--old and/or young depending on the politicians, Social security recipients, etc.).
(3) Voters are
"rationally ignorant."(4) Concentrated benefits, dispersed costs. (see:
this and
this(5) there's more, but my MB would not offset the MC at this point.
Rarely, but sometimes, dictatorships have shown to be better than the alternative. It's rare though. Besides, according to The Dictator's Handbook (p. 1-10, I forget) the current political system provides a greater advantage to incumbent politicians compared to the challengers, so technically, they don't have limited terms. Many incumbent senators and congressmembers have long terms, yet they don't "fulfill long-term promises." Why not? Because it isn't politically profitable to do so.