Good question!
Some possible reasons:
1. lack of training in sign-to-voice (check out how many hours of ITP are devoted to sign-to-voice compared with voice-to-sign)
2. lack of exposure to a variety of Deaf signers (people sign differently; young and old, black and white, male and female, Yankee and Southerner, residential and mainstreamed, lazy and precise, etc.); this is where socialization is important
3. stage fright (yes, some terps are more intimated by hearing people listening than Deaf people watching)
4. lack of experience (many Deaf prefer to do their own voicing, so terps don't get as much practice voicing)
5. too locked into the "glossing" mode (stuck on using the "dictionary" word assigned to a sign instead of using a mental "thesauras" to use words that are appropriate to the topic, setting, and signer); that is, suppose a teen boy chatting with his friends before class, and a middle-aged woman lawyer chatting with her peers before a professional meeting, both use almost the same signs about the same topic (weather, traffic, gas prices, whatever), but I would voice differently (vocabulary and intonation) for each of them; I would NOT "gloss" for either of them
