I think the appeal of exactly those types of games (and, for me, Hearthstone too) is being able to apply adaptive decision making to unforeseen circumstances.

Yeah, losing a 90% shot can be extremely frustrating in XCom but then there's the thrill of avoiding that same shot on your end. Controlled RNG in games help create novel situations that require constant re-evaluation at every turn. The perpetual problem solving, shifting of priorities and "whoa!" moments make it extremely gratifying when that puzzle is solved. When the system is great (like XCom) it means replayability is still there even after hundreds and hundreds of hours.

I totally get why you and many others hate that style of play though. I've thrown many a thing from RNG in these types of games..... but I always come back.