Whilst I don't think a 20 level fighter (or barbarian, or anything) should be nearly as strong as Hulk, I think the official lifting math is a tad silly. I think it should ramp up significantly past strength 18, as otherwise many big creatures are actually pathetically weak, and there's no way to represent truly super strong things. Under the current rules Thor would need strength of at least 7000!
I also think people are looking at the wrong things here. When you think of teh 20th level fighter as "Hulk", its not so much "I can literally lift anything". Its the notion: "I do so much damage that I can rip apart any enemy that comes across me, and I'm so durable that almost nothing can stop me".
This model of the 20th level fighter is: Yes a 20th level wizard can teleport, alter reality, summon demons, etc. But....there is nothing scarier in the multiverse than facing a 20th level fighter directly in combat. That to me is the Hulk or Thor model of fighter strength.
As an example, I have long run a campaign with an organization of high level people. This group has 9th level magic at their command, and yet all agree that the strongest among them is the 20th level fighter. For a long time, my group always wondered why that was, until finally they were in the final fight and got to fight along side this guy.
He was a 20th level champion fighter with max hp, a legendary action, his indomitables automatically succeeded at the save, and most importantly, his artifact weapon (+4) automatically dispelled (or disintegrating in the case of force effects) any magic it touched. No force cage or wall could hold him, summons were ground to dirt at his feet. The party's wizard, 12th level himself who was very powerful and very cocky, looked at the party (who had faced a CR 30 group of githyanki and survived) and said "I would be terrified to face this guy."
That to me is what a 20th level fighter should be about.