Carousels help you to complete your items set and help players that are behind.
Don't compare Summoners Rift with TFT. LoL is 100% tactics and skills.
Today your opponent got FoN, tomorrow you'll get your wished item from carrusel and so on.
Playing more games you will notice that luck and bad luck (RNG) are balanced.
If TFT has, let say, 10% decisions and the rest is RNG then this 10% determine your rank position, never RNG or whatever items are in carousels.
The real problem and hard to balance in TFT is OP composition/synergy/unit. That's why they made set 2.