A lot of the common silvers you see sold at a petsmart or whatever may be hybrids, but for the most part the serrasalmids don’t hybridize that much. Piranhas are hard enough to breed as is, and many will kill each other long before breeding.
Many other fish also don’t hybridize that much, they have to be really closely related to do so. An example is livebearers, where you find molly, guppy, and platy mixes often when they are together.
Then there’s the large group of fish I haven’t mentioned yet…cichlids. With mbunas and Central Americans, half the time you never can be sure they aren’t hybrids, specifically with Midas, devil, and trimac. Convicts will breed with just about anything. And then there are the 2 most famous hybrid cichlids. The blood parrot:

And flowerhorns, which are known for huge nuchal humps, amazing colors, and unbridled aggression.
Both are largely amphilophus and Vieja, but no one is 100% sure what they are made out of.
Most hybrids are considered undesirable because they muddy up the gene pool, or simply don’t look nice.