Not American, but guessing:
- If you do some, there will be nagging requests to do all of them, however minor they would be
- Accents are not a clear-cut thing, hence customers would argue that "this is not how we pronounce X" based on their variation
- Maybe there's no real need, as most companies in need of a TTS probably operate in the whole country (think telcos and such). People are used to the generic accent since the TV and radio appeared.
