I think the Buddha actually made a distinction in between the intention (or volition) and the mental kamma. The volition gives rise to the mental, speech and action kamma.
I think he (or maybe it's just commentary?) also put more weight on the mental kamma than the speech or action kamma.
I think that's something good to contemplate about. Maybe it could be because when one speaks or acts, it's easier to see what kind of fruits that will make... rather than just thinking about it and then not seeing what actually happens if that's acted or spoken upon?
Also, it might be because the mental kamma happens more frequently (just from my observation) rather than speech or action kamma, so there's more opportunity to practice with the mental kamma.
Also, I think if the mental kamma becomes habitual, it will eventually find a way to manifest itself into the physical realm... so it's always good to take care of that every time it comes up. Study it... don't try just to ignore it, or suppress it.