Some clients do have this rule, actually! It is a pretty interesting solution, that seems to work out OK. For example, most Bitcoin cash clients don't accept

Some clients do have this rule, actually! It is a pretty interesting solution, that seems to work out OK. For example, most Bitcoin cash clients don't accept reorgs longer than 10 blocks, as such a reorg is 100% an attack. This protects the network against large reorgs.