The reason for that is pretty simple the runner already has possession of the ball so once he breaks the plane and is in the endzone it's a TD. For the WR he has to catch the ball that's really it, maybe it sucks because of how complicated they make that, but they're trying to balance it for the defense that they don't just give it to them when he didn't even manage to hang on to it. As to the out of bounds part that's just the same catch rule as if it wasn't a TD if it wouldn't count then it's not going to count in the endzone, meaning if you didn't secure it while you're in bounds then you caught it out of bounds, not a catch. 🤷♂️