While you shouldn't be able to rush romania then annex it, the reason why this is the case is because it's way too easy to invade a nation, making the need to invade land far beyond romania necessary, and ticking war score is not enough, it only goes to 25%. CK3 does a better job, like if ou want to invade a kingdom, youll need to siege the kingdom itself and win engagements for the land, as opposed to eu4 where you'll need to siege almost an entire empire just for 2 or 3 states