Energy. Creating a single anti-hydrogen atom requires an absurd amount of energy to first create a collision in a particle accelerator and then capture that anti-hydrogen before it eliminates against another atom.

Only about 0.01% of the energy used to operate the particle collider creates antimatter, the vast majority of which is impossible to capture. All in all, the efficiency of the entire process - if you were to measure it in the e^2=(pc)^2+(mc^2)^2 sense - is probably on the order of 1e-9 or worse.

Nothing we do creates it at any kind of scale, and it's a pain in the ass to store.

Not to mention the only way to create it is with energy (it doesn't exist on Earth), and we can only do so at terrible efficiencies. So even theoretically it's pretty bad.