Because we don't really care about trash DPS. Casters are better at dealing with trash anyway.
Melee exist primarily to kill bosses. Boss DPS is what really counts.
The only reason we bring 8 Melee in Raids is because they have the best sustained DPS to take down bosses. If it weren't for that, there'd be little reason not to bring 12 casters on every Raid. Maybe one tank to hold aggro on some bosses, but even that's probably not necessary.