Where do you get this idea that Urgot has a bad mid/late game?
Urgot is god tier in the bottom lane of Dominion (as is Yorick), and that game is essentially nothing but early game, lanning phase for him.
Urgot is generally one of the worst 3 champions in Rift, where there isn't just a lanning phase.
From that it's easy to extrapolate Urgot's power curve is the same as Yorick's, god in lane, falls off a cliff after lanning is over.
If you compare Urgot to the strong-early Lee Sin, you also see a lot of simiarities. Urgot and Lee's kits give them stats and capacities in pretty much every imaginable place, but spread the scaling of those capabilities around enough that you can only really push ~40% of their kit into late-game with dedicated itemization.
Lee's HP shield scales with AP or armor/MR, Urgot's HP shield scales with AP or Armor/MR. 2 of Lee Sin's abilities scale with AD, 2 of Urgot's scale with AD. Urgot's ult only really scales with plenty of durability, his 1200 range harass only really scales with tons of offense. You sacrifice a significant amount of Urgot's kit as the game progresses based on itemization, and whatever "heaps" of damage Urgot brings into late game is far surpassed by several champions with better AA steroids and longer range.
I mean, sure - Urgot can do things late-game, but with only 425 AA range and a 130+85% AD skill serving as a sudo-AA steroid, Ezreal can hit harder from further range, and you've also got champions like Jinx with a 130% AS steroid. Or on the tanky/bruiser/siege side of things, there's Nidalee and Elise and Gnar.
When opponents just ignore you completely and let you run wild with AA+Sheen procing them in the face, yeah you can do heaps of damage with Urgot. There's just so many champions that scale so much better and apply their damage in more effective ways end-game than Urgot to realistically say Urgot scales well into late game.