I've installed Ikea kitchen cabinets before. The assembly is trivial (they're just boxes), and they all sit on a single rail you screw into the wall at the start. Lifting them into place takes an extra set of hands, but it's not high-skill work. This is not custom cabinetry or anything.
Screwing things into the wall takes some patience, knowledge, a drill and some skill. Not everyone is comfortable with that, or has time to research different types of wall, pick the right fasteners, deal with any surprises during drilling etc.
(e.g. the walls in my (rented) apartment are mostly really solid. Some parts won't even yield to my hammer-drill. Putting up my projector was easy (hollow wall) but I spent 2+ hours putting up my projector screen on the opposite wall (even though only 2 screws).
The hard part is most houses do not contain a single plumb, true, and square room. Walls do weird things, especially where they meet floors and ceilings. Floors are hard to get flat. Etc. I'd say once you resolve all this and get your rail level and parallel to the floor, yes, then throwing up the cabinets is the easiest part. After that, you probably want to learn how to do some molding to trim them in.
Having someone else install your IKEA cabinets for you probably won't help with the quality of the installation if your house is a bit wiggly around the edges.
Perhaps not, but the hope is someone who installs this stuff for a living has encountered these situations enough to know how to handle them, whereas the average person who might try to install this on their own is likely encountering that for the first time.
