Because they do resemble the actual outcomes. The 2 major parties in the UK regularly get majorities in Parliament on far less than 50% of the vote. In the USA gerrymandering has been brought to a fine art. How can this not contribute to discontent, when the majority of people get a government they didn't vote for?
It’s not like they’d be content with a deadlocked parliament where the ‘government’ they voted for can’t enact any policies. It’s good that there’s two dominant parties, both fairly central, who continually moderate their policies to keep high levels of support, and after an election, one of them gets to actually govern.