I've got a player with Gauntlets of Ogre Power +2 who just wants to pay to increase them to +4. I think part of the reason is he has a belt of healing, and doesn't want to use up his belt slot.
I know in 2nd edition there were Gauntlets of Ogre Power, which were less powerful than Girdles of Giant Strength, though they stacked, IIRC. I'm tempted to enforce the concept of +4 and higher bonuses only being in belts, but will not impose a wealth penalty on the selling of the gauntlets to fund the belt.
But, in giving him my answer, I'd like to tell him more than "it's D&D lore as to why it is that way." That will still essentially be what I'm telling him, but I'd like to give any backstory on why it is this way. Anyone know a reason more than what I've already described?