Because EQ is an interlocking system.
If you shift the EQ emphasis on one sound, then you are changing it`s interaction with another, and then may have to EQ THAT sound to compensate, and then that effects the harmonics of another sound, so you have to EQ THAT sound.

No it`s way too much bother. It takes me at lease a day to EQ and mix one track. At least, to then start really screwing with that is madness.

I use EQ automation very rarely for certain tricks, but it`s something that really screws your mix.