Good observation.  And a small religious party in Israel which got 3% of the vote and has 1-2 Parliament members can walk away and cause a collapse of the government. It causes instability.

2. Furthermore, a PR system gives a huge amount of power to the unelected internal party leadership.
Party officials who get to do backroom deals have disproportionate amount of power.

3.  Your economic or social or political program gets watered down, because you have to compromise, give-and-take.  The alternative is you let the other guy implement his policies and if they don't work, you win the next election and get your shot at it.