It's the natural state of the UK political system. The PM has a (figurative) gun on his head constantly, and has to fight and defend his actions in parliament almost every day of the week. Keeps him on his toes and ensures that he gets replaced if things go down.

The stability of the US presidency comes at a cost. It allowed a senile president to stay in office when it was clear to everyone that he was incapable of assuming his function. Or with the current presidency, it enables a president to renege on pretty much all of his electoral program with impunity. Both would have been replaced 6 months into their term in the UK.