Because the FTC is budgetarily starved intentionally and doesn't have the time to go after every minor false claim out there. It's also a somewhat losing game because all they really have to do is have a single measurement showing a car did produce more horse power after placing the slab on the chip, which is extremely doable because measuring generated horsepower is far from a perfectly repeatable measurement, and they could slip out of any case brought by the FTC or winnow it down to a minor fine for less than the profits from their sales.