Kog'Maw is a hypercarry. Why build him like a mid- game ADC?

I just don't think the Tri Force build is all that good on Kog. Sure, it gives him some mobility (you could just as easily get an early Furor Enchant), and Tri Force gives you some decent up front burst, but Tri Force's sustained damage pales in comparison to the traditional hypercarry build - Phantom Dancer + Infinity Edge.

Also, let's talk about BotRK on Kog. I don't think it really has very good synergy with his kit, and ESPECIALLY not with Tri Force as a first item. The only thing this build gives is decent mobility, and decent up front burst (if all you're going to do is get 1-2 Tri Force autos + BotRK active off, but in that case you're much better off with a real burst ADC like Graves). But the thing about BotRK after Tri Force is that it's synergy is dubious. BotRK does % of CURRENT health, so why would you want to pair it with a burst item like Tri Force? The on hit damage is just going to get worse and worse the longer you're hitting the same target. Whereas with the traditional Crit build, you have a nice, smooth and consistent damage output (which is nice when you're trying to calculate your damage). You also do more damage, period.

I just don't see the point in delaying your power spike SO long, just for a set of items that hardly even synergize with Kog's kit that well. BotRK has poor synergy with the burst of Tri Force, and Tri Force sacrifices sustained damage for some mobility and some meager burst.