They’ve found in studies that republicans are the more polarized of the two. Essentially they’re more likely to oppose or support their party blindly. (I believe that was proven with trump but they’re still releasing data on it).
And I’m not saying that people completely don’t matter. But that in experiencing both systems, which you haven’t. People matter more in presidential systems. Because I’m parliamentary systems the leader plus the party line determine most of what’s happening. Even in Britain.
Also I don’t see fifty UK mp’s gunning for the prime minister position or jockeying for firer favor at it the way a presidential system encourages such behavior.