Why do metal players insist on harmonising riffs and solos when it just makes it less heavy?

I'm sick of good riffs being ruined by the obligatory 3rd harmony notes being added. And as far as melodies/solos go, using harmony is just a way for them to get out of having to really write an effective line as adding a harmony note distracts the ear from listening to whether they're really playing a good line to begin with. But I digress, my overall point is that harmonising makes things sound 'nicer' which to me just makes it that much less heavy and takes away 90% of the impact a riff or lead line could have. All the heaviest riffs are straight to the point, no excess frills.