Because "raise the question" doesn't have the same connotation as the commonly-understood meaning of "beg the question".
Begging the question means (to most people, I think) that there is an obvious question in response to the statement. The statement itself is "begging" for this question to be asked.
Raising the question means that one's response to the statement is to ask a question. It may not be obvious, and it's not an attribute of the statement itself that implies this response.
That's a valid point. But isn't it a bit archaic to have the thing begged for as a direct object? Wouldn't it be more normal to say `beg for the question'?
Yeah I can see that. But it feels weird to say it.
The English expressions I do wish people would get right are "moorish" meaning spicy (after the Moors), not "I want more of this", and "enormity" meaning horrible, not huge. But English is always changing, and using these phrases in their correct meaning now confuses people.
