And Yes natural hybrids do occur but they are are still within a locale or region. And they are unlikely to be common or reenter the gene pool. If you have any knowledge on the natural hybrids you would know that they are not being kept in vivs and bred. The frogs also fair just as much chance if not more to breed with their own species in the wild which is not offered by these people in captivity. As you said the regions and territories do cross. yet many of the hybrids are left sterile and do not continue the gene pool which keeps things in balance,
When you throw a few frogs of different species in a viv its nothing more than the interest of creating a fad or profit that results in many more frogs that flood the market.

Michael