Up until about 10 years ago I would have agreed with you. I've been building my own computers since the 90s and I've bought about $100k worth of various computers for companies over the years with more on the horizon. My first computer was an AMD K6-2 but my dad got me into computers with a custom 486-DX.

Lets look at some PCs I've built. An AMD FX-9590 with dual 570HD GPUs in SLI. That's 700w right there. And it was impossible to keep cool, sounded like a vacuum cleaner, and felt like a space heater when you walked by. Not cost effective or efficient. Motherboard failure, but not before one of the 570s quit.

I then had a first gen Ryzen. I had so many HDDs that I had to have dual PSU's for a total of 1,850w, but not redundant like most servers are. That eventually failed due to motherboard failure. Also lots of drive failures. Way more than an OEM box. And when they failed I had to take the system offline and tear it all apart. Modern servers have hot swappable HDDs and even PSUs... so you can replace these parts without tools and without bringing the server offline.

Now lets look at a Dell R700 series server. Dual, redundant PSU's, ECC memory, redundant systems everywhere, remote management that consumer boards just can't compete with, fit and finish is perfect, drivers just work, and if it doesn't you can call someone and they will make you whole again without "troubleshooting first". When a PSU in a server fails, the server sends you an email. When the server needs a hard reboot, you can reboot it from the BMC even if the OS is panicking. There is a little screen on the front that changes colors when there is a problem, and there are color coded POST error codes.

You usually hear the argument "build your own" from hobbyists and gamers, and I do fit into that category typing this on a full tower Ryzen with a RAID 5 array for all my games. However, the broad statement of " Custom built PCs are overall more efficient, cost effective, and reliable" is simply not true. It doesn't make sense for (most) IT Departments to make, build, or custom order PCs for pretty much any reason. Even when I managed a factory network, the engineering machines were Dell Precision Workstations and they rarely ever had problems. If they were using 10x custom gaming computers I guarantee there would have been problems all the time. Engineers aren't gamers, and most aren't "custom PC" people. The tolerance for problems is very, very low. Time is money. You do a lot of your own troubleshooting that people at work doing their everyday jobs aren't going to be willing or able to do on their own.

The IT Deparment would rather a PC with 9,000 CPU marks and 2 support tickets than a machine with 12,000 CPU marks and 25 support tickets.