I’ll answer your last question: because the GOP is not united. Some GOP senators / reps prefer to look cool by obstructing the President as opposed to working with him to move the agenda forward (I.e Flake, McCain, Collins). We call them RINOs, republican in name only. Oh and don’t forget the failure Mitt Romney and his desperate yet fact-less swing at the President.
Political beliefs aside, establishment politicians on both sides are terrified of Trump because he is challenging the status quo. These establishment politicians have remained in power for so long that any changes in the status quo threaten their power and job security.
Trump’s personal style/tweets/gaffes aside, at least he is an honest pragmatist.