Why do I always see Catamarans without trim tabs??

Some do.
Some run one big one in the center..."tunnel tab"

MTI's don't need em. There are cats with trim tabs...Nortech's, some Skaters, I'm sure plenty of other brands have them.

They don't need them. They basically have two hulls spread out 9 + feet so they dont need to drag tab to help keep the boat stanle and from chine walking or listing side to side etc. Some cats will have a center tunnel tab but unlike trim tabs or planes that use the water the adjust ride. The tunnel tab controls the packing of air down the tunnel to make.it smaller if needed and help lift the boat out of the water even more if it's hopping in a certain speed range

I have a friend on his second spectra and both had a pulsing hop and he is now pitting tabs on it to take that out. I have run with MTI's that had the same hop ?
Rich

Not all cats are the same.
Single cats will often have tabs.
Twin cats will often have tunnel tabs.
Some cats have center pods.
Some cats do not.

It all depends on each boat and it's power package.

Tunnel tabs can be dangerous, I had on on my DCB which I broke last year. I discovered the boat ran just as well without it and have not repaired it yet. The only reason I really need it is to act as a step to get in and out of the boat (in the water and on the trailer).

I have a big Dana marine twin ram tunnel tab on my cat. I rarely ever use it. It makes for a great swim platform to get in and out of the boat. My boat doesn't hop at all. I think with the length of the tab, it makes the tunnel longer causing the boat not to hop.

The fact is, not all cats are the same. Some cats will have a center tunnel tab but unlike trim tabs or planes that use the water the adjust ride

so it isn't a tab, it is a flap. adds lift and drag when lowered.