Imagine there were 100 constituencies, and in every one party A secured 51% of the vote and party B got 49%. Under our current system, party A would have all 100 seats, and party B would have no representation at all; whereas under PR, A and B would (simpisticly speaking)split seats 51-49.
Worse, imagine that in 51 constituencies A gets 51% of the vote, and in 49 it gets 0% of the vote. This gives A a majority in parliament even though voters as a whole prefer B nearly 3-to-1.
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.