Because when going against Riven in a melee meta you'll pick something equally dumb.
Fuck Riven. She actually made me pick up Gnar as my new main.
Gnar is so good vs. Riven, I haven't lost a match vs. her yet. Just E away when she tries to come at you and poke with QW until she wastes her mobility. The only other option to approach is to flash in, but who would be bad enough to get caught by that? Gnar is just good in general. Brutalizer + Sunfire + SV = One tanky mofo. I always like to build into Ghostblade with him because of the attack speed boost to proc my passive and the armor pen because Gnar doesn't AD scale as well.