Decks that run a lot of counterspells already compromise a great deal to run those counterspells. To cast FoW without hardcasting it, you have to exile another blue card. To cast Mana Drain, you have to leave up two blue mana, with which you could cast ponder, brainstorm, whatever. Most counterspells are costed in such a way that, if the caster is not very discriminating in what to counter, they run out of resources very quickly. Aggro and combo players have many ways to outmaneuver, and many occasions to out-luck these control players even without uncounterable cards.
With the introduction of uncounterable spells, lands even (not even 'legendary'), players can cast stuff with one less thing to consider about what the opponent might have in their hand. With things like Cavern of Souls, you don't really have to consider counterspells, so there's one less axis around which strategies can be made. If you're playing against blue, you'll win more often, but by having these cards in play, the game has been simplified. Turn 2 Extirpating Life from the Loam against a Lands player entails no suspense or strategy. Turn 1 drop Cavern of Souls, dump a bunch of moxen, play Glowrider/Thalia, Reality Smasher, etc., you might as well be playing solitaire if the opponent is playing combo or control.
As a game involving chance, Magic needs to consistently sustain the feeling of 'if I had drawn this, I could've beaten that,' to make you want to shuffle your deck and play another match. With all these extreme cards without possible outs, that feeling does not take place. Instead of playing rock, paper, scissors, playing with these uncounterable spells feels like I can only play rock, and the opponent can only play scissors for the entirety of the game.