Because you need 1 more point in the first tier of the offensive masteries to get the CDR and armor pen, and you aren't getting exhaust or smite. You can put the 1 point anywhere but it won't benefit you at all.

Why take 1 point in archmages savvy? It increases AP not Mag Pen. Since his Abilities depend only on AD, there's really no point is there?

Sheen helps greatly due to his low cd on his q which is a melee ability. He also has very good base attack damage.
And no, I don't expect you to get all the items. You'll prob end up at the warmogs-atmas stage when the game ends. If they buff yorick he'll prob have a better farm, but until then idk.

the build seems good but i didnt understood some things.
Why sheen?
and do you have money for all that stuff with a bad farmer as him?

There's a thread on the lol forums saying yoricks getting buffed/tweaked during the next patch.
I'll update this guide if need be.