I think this is just an aspect of the writing style of that era and where technology was at that era.

The futurist writers (Roddenberry and his staff) probably didn't have a in-depth knowledge of natural language processing. The type of processing the computer does in Star Trek just seems like a natural extension of input/output. You tell the computer what you want and it gives you a result.

From a computer science and engineer perspective, this is an incredibly difficult task. You have to parse the crazy complex natural language of humans and turn it into a series of procedures and queries to give you a result.

I mean you'd need a crazy super-advanced beyond-data AI for universal translators to work too. .. but that's just a convenient plot/suspension of disbelief device. :-P