Because you need health for some more utility and you can take more damage from the enemy.
Scaling Magic resist because u dont need that much magic resist at the begining but lategame the the enemy ap carry is starting to do damage you really need it to stay alive.
ability power and magic penetration... well you know what you need it for! To get all the kills and to pwn the other team!

This is for a mixed build. Its offensiv when you want it to be offensiv and deffensiv when you want it to be deffensiv.
You gotta stay deffensiv at the beggining(level 1-6) after that you can play as aggresiv as you can. When you are getting ganked you need to use Twilight Shroud Emidietly and the run away when its still active. Lategame (when all are in level 18) you have to be on the ad carry right away and its easy becaus you can jump that champion without anybody stopping you. After u have killed the Ad carry you should go for the easy kill like mid or jungle then atlast you should go for top then support
(Ad Carry, Mid, Jungle, Top and last the Support(top or jungle depending on who's easy to kill).

This build is for every type 5v5 game you can use it in blind pick, draf pick and ranked. I know that the Lich Bane gives mana and that Akali dont use mana but you really need the ability power and the Spellblade Buff. You need to buy the wards when you have 75-150gold left after buying your item or items.

Because they are cowering all the damage areas and utility against your opponets.