Why would I ever pick Malphite?

Lol. Maphite has literally THE STRONGEST ULTIMATE in the whole game. What do you want? Shorter casting time for maphite ult? 10 sec cd for maphite ult? Increased knockup duration? Yes zac is a little too strong at the moment but why do you compare him with malphite?

Instant cast time, much faster travel speed, as well as it being un-interuptable (you can really piss off a Zac if you're Trundle) gives it a lot more power than Zac's ability. And for a potentially team-fight changing amount of damage and CC it brings to the table, it's worthy of being an ult.

Unstoppable. Faster. Bigger.

Malphite can punish split second mispositions in a way that Zac never can. Yes Zac has great engage but it requires set up and it isn't near as game changing as a Malphite ultimate.

because malphite is the best champion in the game

why should you pick malphite when has a bit slower knockup than malphite on his W when can combo the same effect.

Why would I ever pick the option to use my brain?

If you're going to compare Champion abilities, at least compare the entire kit, and not just one select ability per champion. (especially when you're comparing an ultimate and a basic ability with few similarities to each other.)

Because he's rock solid.