Elise Recommended Build Q's

I can't make sense of why the recommended items for Elise seem to prioritize Magic Penetration over Ability Power, and why Attack Speed isn't more popular on her?
is recommended as an offensive item but I feel like that would only be optimal if you stomp the first 5-10 mins and build it immediately after
.
I wanted to try something else out so I went into a game with Ad/atk speed/armor/cdr runes, built
(before game ended) and felt like a magic
spider. I felt like it was very flexible and I could have built more damage or gone much tankier and been effective.
I am no itemization pro, but I have been trying to put more thought and research into my builds. As such, I am skeptical of this idea myself but it seems cool to me and was curious to get others' thoughts!
is a significant amount of damage and durability for a pretty low cost and scales rather well throughout the game. After you have those items, you can easily build offensively or defensively as necessary.
) The stickiness and damage reduction from that combo, along with CDR, are hard to pass up, even though she is a highly magical champion.