Well yes, that's the obvious solution, but:
1. We are always under massive pressure to reduce our prices. Boss won't let us cut our daily rate much, so guess what happens...
2. If I go to my boss or even the client with a proposal that includes 1 person-week's worth of time just for "replying to e-mails and general admin", he will throw a fit. They sort of expect these things to be done invisibly behind the scenes. Like responding to a constant flood of client e-mails and admin "isn't proper work" or something.
3. Our clients are just plain tight. Unless we quote absurdly low, we don't get the work and it goes to some a-hole contractor who doesn't have any overheads.
4. Even if you do try and estimate things properly, it always takes longer than expected. I think there's a phrase for this, "something's law".