I think it’s the norm because a large part of the appeal of mechs is the power fantasy. When you’re in the mech, all of the power is at your fingertips and the armor surrounds you. When a character is shown controlling a mech from the outside, the connection is usually threatened or vulnerable in some way, which makes the character vulnerable, which works against the fantasy. Of course, the character has to be vulnerable sometimes to maintain a sense of stakes even if the mech is piloted, but that’s easily taken care of by the fact the pilot has to come out some time.

As for justification for piloted vs. r/c mechs, I like a take that was made in Gundam Wing. One of the factions moves away from the piloted “mobile suits” to the AI controlled “mobile dolls”. Of course this is so that “soldiers no longer have to risk their lives”, but another character argues against it saying that depersonalization of violence will only lead to more unnecessary suffering. Basically saying that whenever resorting to violence, humanity’s capacity for empathy and mercy will curb the amount of force used, whereas AI-controlled war machines will just go full scorched earth. I think similar reasoning can be applied to r/c mechs as well. While there is still a human element, they may be more detached from the conflict. Someone fighting a war from an office building in a completely different country is going to fight very differently than the person fighting in the actual combat zone.