I bought an OEM pre-built PC in recent memory (not a refurbed one to take apart and re-build and sell as a gaming PC, I'm talking about just buying it as-is and using it). It was that "HP Power Gaming PC" that was sold at Walmart B&M stores back in like 2018 or 2019 BF-esque (maybe older, maybe 2017).
It was cheaper than buying the parts, at the time, heck, the GTX 1060 3GB (which turned out to be an inferior OEM-specific version), and the i5-7400 CPU, were alone mostly worth the purchase price. Plus, it was easily upgradeable to a SATA SSD as a boot drive, having a spare SATA power cable and SATA drive bay. Also, the factory PSU was a Lite-On 80Plus-something, with dual 6-pin PCI-E cables (only one used by the GPU, but there were not dual GPU slots).
So all in all, not actually too bad. Benchmarks, were excellent. Gaming was pretty decent, according to YouTube reviews. But they also touched on the Dark Side of OEM PCs... LACK of upgradeability. There really wasn't much that you could upgrade to the core component. Yes, you could swap out the video card, and yes, you could swap out the HDD, and/or install a SATA SSD for a boot drive, so that wasn't SO bad. But as far as re-using the case, and slapping another mobo in, not really going to happen.
Custom PCs, IF YOU UPGRADE AT ALL, are WAY better, IMHO. Even if you could physically upgrade an OEM pre-built, to a new platform (new mobo/CPU/RAM), the OEM Windows license doesn't transfer. So you're NOT saving any money, if you are the upgrader-type, by going with an OEM pre-built as a base, just to get the Windows license included. (*)
If you instead, purchase PCs as if they were consoles, and upgrade in completely discreet PC steps, meaning, you buy a new PC, and completely replace the old every time, and never do partial upgrades, like you can and probably should with a custom-built rig, then I guess I can understand the logic behind it, but I don't agree with the reasoning.
(*) The exception to this rule is, for really budget gamers that can't afford or don't want to spend money on a full custom build, you can pick up off-lease corporate refurbs, mini-towers are best, that come with a COA on them with a Windows product key, and then just slap in a lower-powered GPU like a GTX 1650 4GB, that doesn't take PCI-E power, or if you were lucky or judicious with your choice of OEM system, you might have an ATX-compatible PSU connector and PSU size, and you could possible upgrade the PSU wholesale, to accommodate a better GPU.
But building a newer, custom-built rig is almost always better, longer-term, if you have the budget for it initially.