Cho'Gath has a tremendous amount of CC, utility, is an extremely powerful jungler, a monstrous(no pun intended) health boost, high base run speed and his Feast scales directly from AP, and does increased damage to minions.
Garen has high mobility, inherent tankiness, sustained AoE and a silence - and while he is a weaker champion than Cho'Gath, he still has strengths.
Darius has a short range grip, armor penetration, an AoE and a slow... and his ult which requires him to hit you 3 times with auto attacks to do any real damage. He has a very slow base movement speed, and very little utility. He is very easy to kite.
Darius was picked 6% of the time, banned 6% of the time and had a 0% win rate in recent tournaments. He is far from a tournament viable champion. By your argument's logic, Nasus would be nearly 4 times as powerful as Darius which is obviously not true. Darius is a champion whose inherent, intentional strengths are accounted for by designed weaknesses.
In fact, his quote, "Exploit every weakness" sums it up quite well: kite him, harass him, and don't let him run top lane. He will be useless compared to many top lane champions mid game, and late game teamfights are typically decided too quickly for him to be of any real consequence.