You don't have to build very many PCs before finding yourself with a bad power supply or motherboard. (In my experience these two are the most common to be non-functional out of the box.) This can be frustrating enough even for someone who is experienced.

Or incompatibilities or drivers/firmware that need updated or... There's just a lot that can trip you up and I've run into a lot of quirks and issues over the years that--if I were just starting out--I'd just assume that I must be doing something wrong.

You need to know (and trust) those things. IKEA is a common brand and they deliver everything you need in 1 box or so. Biiiiiig difference for the common man.

If all of your parts are good from the start, I agree. But if you are building a computer for the first time and get just one bad part, it can be difficult to isolate where the problem is if you don't have a box full of spare parts you know are good that you can swap out one by one until you find the bad part.