Why are some mages cc nerfed when they depend more on them?

So lets take karthus, swain, veigar. A while back, karthus w was nerfed to the point where its pointless to use it early. However, karthus relies on slowing his opponents to have a much better chance of dpsing them down with q.

Swain's w was nerfed to the point where is also useless and shouldn't be casted in a straight up fight. A mana cost of 80 for 80 + 70% ap, or ult which does 50 + 20% ap and costs 50 after 5 seconds is much better, considering the risk and reward that his ult will do damage, and his w has a high chance of missing. A 1 second snare reward for landing what i consider to be the hardest ability to hit with its insane delay is not right. Some people don't even care about getting hit by it because they can still easily escape swain by walking away. Unfortunately, the other half of your damage is your q which people walk out of pretty quickly.

Veigar's stun is changed to the point where its impossible to land and you lose pretty much all of your damage without landing it.

These champions have a lot of their damage based on landing their cc, but their cc ability is nerfed to the point where its useful in limited cases early. Most of the time its better not to cast it just because the mana cost already outweigh the benefits.

Yet we have cassiopeia, syndra, ryze, viktor. The viable mid laners played in world?

Cass cc is a huge area damaging slow that prevents movement abilties. Yet she doesnt even need it to do the damage she does. way better than karthus w.

Syndra is a fairly far ranged stun, but she doesn't even need it to output her insane damage.

Ryze has a click and point snare, i would say he would need it to land a ton of his q damage, but his cooldowns are already so low. way better than swain's w.

Viktor's slow is not the worst, it is a threat which you have to walk out of and it is still a slow. He doesn't really need it to spam lasers to wave clear and do a decent amount of damage. Basically a veigar cage but slows and then stuns inside of it.

How come some champions have much more powerful cc abilities and can still do a ton of damage without it, while other champions cc ability gets nerfed and they lose a ton of damage without landing it?