Why do only some anime series translate on screen text? Some anime series use captions to translate signs, text messages, speech bubbles, and the like while others do not. I'm left guessing when some characters only communicate through these messages and nothing is translated. Are these extra translations done by specific studios or on a case by case basis?
In some anime the additional text is there just to provide info. If context is provided, the translation usually isn't necessary. However, anime or at least their source videos that are meant to be dubbed and shown outside of Japan, are sometimes supposed to be translated, because most viewers never know nor care about the original language used in production (kids watching TV?).

Otherwise it's just about speed. Some fast-subbing groups only provide the needed dialogue, some groups take their time and translate everything to help the viewer.

Sometimes scribbles and text on walls provide additional humorous inputs (e.g. Sayonara Zetsubou Sensei) and some people like to thoroughly go through them...

Overall it's all about the target audience :)