First you have to choose components, which takes time given there are so many choices. You have to make sure they're compatible - for example my cooler wouldn't fit into my case. You have to make sure everything works - my case had a faulty power button so it took an extra couple of days and some hassle to get a replacement.
Then you have to put it all together. Mounting the CPU with heat sink paste is ok, just annoying. Mounting the heatsink to the CPU is always a nightmare. Then you have to work out whether to put the motherboard in the case and then plug in the power cables, or if it's a tight space you plug in the cables then put the motherboard into the case. RAM has to be mounted, video card too.
Then you have to plug in all the power connectors, sata cables, and jumpers. If anything doesn't work you have to diagnose the problems, which for me required a mutimeter to work out the power button on my case was faulty. It was at this point I also discovered my heatsink was the wrong size, so I had to repackage and return it. That meant taking the motherboard out, of course. You have to think about airflow, which means tying up cables, and making sure things stay cool and quiet. This is inside a relatively small case, even if it's a mid tower, which has plenty of sharp edges.
Next you have to install the OS and dozens of updates. Sometimes things go wrong, and you have to do a new OS install. I had to reinstall the OS once. Then you install all your software, antivirus, etc. Next run prime95 or something to make sure the heatsink is mounted properly and the CPU won't overheat.
I found my CPU fan/heatsink rattled, so I contacted support for Noctuna, who are mailing me a support bar to help stop it. I'll probably have to take the motherboard out to attach that, which means taking every single connector off the motherboard then remounting them all again later.
This was the third time I chose components and built my own PC. It took a couple of weeks between ordering things, having them arrive, getting replacements, installing everything, and getting it working. I could've done it more quickly, but I wasn't in a huge rush luckily.
So you can go through all that, or you can just buy one already made and save yourself the hassle. Technical people might enjoy it, but 99% of people should just buy something preassembled that just works.