Opportunity cost. Sure you can save a lot of money by doing it yourself, but getting someone else to do will also free you up to do other things, like spend time with your family, sit around and admire your tank (or curse your tank).
Not everyone can afford to burn that kind of money on a reno, but not everyone can burn those kinds of hours spent on a reno either.
At any rate, perhaps look into the middle of the road options as well. That is, instead of getting one person (or company) to do the whole thing, break it down into parts or smaller projects like, framing, drywelling/mudding/taping, electrical, etc, and maybe do the smaller easier projects yourself while hiring someone else to do the bigger "projects".
Of course, if you can afford to have someone come in and do the entire thing you mind as well
