Why does Akali have so much sustain?

I've been hating playing against an recently because they're just so damn hard to kill. Innate sustain in lane along with a free poke that leaves a mark that lasts longer than the cd of the ability. Can just freely Q spam you until you're nice and low and uses her built in flash/invisibility to close the gap and delete you pre6, and that's the easy part of the lane. The real hell is when she can just Q you and all in you, using shroud to get cd on q back to make sure you're dead. How are you supposed to kill her when she rushes so all the damage she does to you, she heals along with her passive that already gives a crap ton of heals? I've seen an akali heal half her hp with just one auto on a marked champ thanks to it. Grievous wounds doesn't seem to do much against it either. Is there something I'm missing to fighting her, or is she just plain broken atm? Help would be greatly appreciated.

Also being an assassin why does sustain make sense on her kit at all? Only other sustain on an assassin would be on and his is only if he kills the target. Not only that but it uses mana (a fixed resource unlike energy), and only heals 20-71 with no ratios to increase it? Akali's is 5-60+(.6 ad)+(.45ap) and is free.