Dowding, most of our politicians are lawyers. In the USA, law is an adversarial system, with each side trying to "win" at any cost, by any means. When lawyers become politicians, they apply the same thinking to government, with their main objective to "win". So, we get "party line" votes where 100% of one of the parties votes to support their "leadership", and dissention is punished within the party by witholding key committee positions. Compromise is rare enough to be newsworthy, and is openly disdained in campaign advertising. The "other side" is painted as not merely different, or even misguided, but as "evil". Voters are left with the choice of the lesser of two evils.
Sometimes, I think a parlimentary system, while probably no more effective, might be at least more entertaining.