Why am I seeing a lot of fizz with AD builds?

because a professional team used it in one of their matches

The %HP missing magic damage that was a Flat dps function of his W passive, Became an On-hit Function with it's active. So he scales a bit better with AS and synergies with BotRK. This is Obviously at the cost of his AP burst, but this play style accomplishes what people need from it.

Scaling pretty much means nothing in most cases, or else many other champs that are often AD (i.e Shyvana) would be seen AP more.

It's just what became popular after the recent changes that had been done to his abilities, and after it was done in the LCS as well.

W scales by itself with it's % damage.

Q and E have useful utility and decent base damage even without any AP

R is a CC and damage amp.

Jax's abilities also all scale with AP.

AP Jax OP?