Ask a biologist why predators don't exterminate all their prey, and part of the answer is often that there is an ongoing arms race between predators and prey, with both parties continuously evolving new ways to cheat each other.
The reason for the morphological limitation must be found in how bats emit their sound. Most bats call out of their mouths, and this allows them to emit loud sounds. Many gleaners, on the other hand, emit sound with their noses, and this makes their calls 20 dB lower.
Can you imagine screaming through your nose?