Why can't people have the same name?

A common reason for most games that have this rule is "too many players". I'm not sure exactly why though.

Anyways, they don't really have much incentive to change that, because many people purchase name changes frequently (so if it makes them money, it's staying).

An alternate "solution" for this could be display names (take Roblox for example) but I can't see them implementing this, to be honest (it might take away incentive to pay for name changes).