I think we have a number of different phenomena going on here.
1) You assume that tongues is incoherent and thus random and so should not have sounds or phrases which are repeated for years by a large number of complete strangers.
Isaiah 28:11,
Acts 2, and other verses clearly indicate an actual language, but one that was not studied by the speaker.
2) Individuals seem to have their own distinct "prayer language" and seem to repeat the same phrase everytime they speak in tongues, but that distinct pattern is unique to that person
3) Some people do fake it
4) You assume that since some...many?...most?..are fakes, then all must be
Applying the same logic, because some religions are false, all religions are false. Because most doctrines are false, all doctrines are false. Because most gods are false, God is false.
I would say this is an incorrect assumption. There will always be some who attempt to imitate the genuine, but that only cheapens themselves, not the genuine.