Firstly, the Prime Minister and an MP are very different, so it’s not really a fair comparison. Replacing an MP with one of the same party might result in what? Your bins being taken out on a different day?

Anyway, I think this is a “don’t let perfect be the enemy of good situation”. Without any safeguards, an assassination is most likely to come from someone across the political spectrum than someone next to them. So it makes sense to focus on preventing that even if it does open a potential (risky to execute) exploit.