Why do assassins/stealth champs have so much survivability in their kits?

Maybe assassins wouldn't be such a problem if it was actually possible to focus them down and kill them. Unfortunately, due to all the sh*t that's in their kits, this usually isn't possible.

Ahri-3 dashes and healing from her passive Akali-Free spell vamp, stealth, and 3 dashes Diana-Has a shield and a dash, but needs an enemy to use dash Evelynn-Stealth, can remove slows, and gains a huge shield from her ult Fizz-Ignores unit collision (minions won't help you when he tower dives), takes less damage from auto attacks, has a dash, he has an untarget ability, and a slow (possibly the most loaded kit of any assassin) Katarina-Her shunpo is basically a Lee sin dash. She can jump to wards, allies, or enemies with it. And she also takes reduced damage after using it. She also gets free movement speed from her 'q' if she hits an enemy, which isn't very hard to do since it's a point and click ability. Kassadin-Reduced damage with passive and ignores unity collision, has an AOE slow, and a teleport on a short cooldown. Kha'zix-Has 2 slows (1 from his passive and 1 from his 'w'), his 'w' heals him, has a leap, and gets free movement speed when he ults.

I don't feel like listing every single assassin/stealth champ in this game. But from the ones I listed, you can see how easy it is for them to survive with their loaded kits.