One reason is "who's to blame if there's a leak"?
Another reason is that to prepare all the pipework without any soldering 'as you go' will make it next to impossible to do. Sections are usually cut and soldered and they become 'fixed' and then work further down the line does not cause this first section to fall apart, which it will if not soldered together.
Unless you've done this type of work you may not appreciate what I'm saying

Another reason is that a plumber will charge you for his whole day whether he's just soldering together your pipework or cutting and installing it himself.
If you feel that you can do the pipework, why get a plumber in at all?