Bush, McCain and Romney all had too much of a "Go along to get along" mentality: at critical moments they would all FOLD and cave to the democrats.

We have a federal government that divides power between the Presidency and a bicameral Congress with a filibuster rule in the Senate. The Republicans haven't had a filibuster-proof trifecta since (checks notes) the 1920's. Most Presidents and Congressbeings want to actually achieve things while in office, and while there are some areas where you can do things without engaging the other party, most anything of consequence requires getting enough votes from the other party to get out of the Senate.

Unlike Parliamentary systems, in our system of government the opposition party almost always has actual, real power in the system. That means it's hard to "unapologetically" push an agenda forward. At some point, you need someone from the opposition party to vote with you. Which means you have to fold and cave to the opposition party. Whether that means losing your public option or passing a clean budget resolution instead of your wish list, you can't get 100% of what you want.