Renekton getting the upper hand against two opponents is much more difficult than it sounds. Yes, Renekton has a support helping him out, but bad positioning where he's in the zone of threat for both enemies while his support isn't means that he would take an incredible amount of harass. Duo v. Duo lanes generally encourage stalemates over aggression due to overlapping ranges of threat, and help from the jungler doesn't improve things much since a 3v2 is not much more pressure compared to what happens in a 2v1 gank.
Assuming the lane phase even goes as well as he hoped and you get that early tower, the enemy adc can just freeze lane at inner turret and farm in an area where Renekton would be really deep in enemy territory and the enemy jungler and mid can collapse on him. Renekton may only really set his opponent back 5-10 minutes of farming while sharing his xp with the support means that he'd be underleveled and unlikely to successfully pull off a roam to mid lane with ability damage that is already falling behind.
When Renekton goes top lane for a 1v1 that he can win, then he gets much more xp from each wave, ganks from his jungler are pretty much sure things instead of gambles, and his lane opponent gets shut down harder. When he pushes down that turret or just feels the enemy jungler could gank him, his higher levels and higher damage on the abilities he so very much depends on can roam to take down other objectives and get the rest of his team fed, too.
