That was maybe once true. If you're literally starting from zero today and buying new parts? I sort of doubt it.

(Though I don't really disagree that it's generally easier to swap out parts on a (home-built) tower computer than it is with most things because of standards.)

It's perfectly true today if you ignore the recent price hikes which happen under different forces (silicon shortage, crypto stuff). An OEM might be cheaper because they will put in lower quality stuff you'd never buy yourself (the absolute cheapest part that fits the bill, the cheapest motherboard, the worst thermal paste), and they have to cover extra services, warranty, building and distributing the systems, etc. I think you can still get cheaper and/or better systems by yourself.

Comparisons are a bit hard because (perhaps outside of some specialty--and more expensive--gaming systems) what you build yourself is arguably "better"/more optimized than a random HP or Dell box. At the same time, the percentage of people who care about optimization at that level is pretty small. I built/upgraded PCs for years and now I pretty much have zero interest.

But again to the basic point, it's certainly close to a wash which wouldn't be true with most things.

It's definitely true for PCs. At best you can get the same price for pre-assembled but usually it's more expensive.

Not sure about laptops though.

Admittedly it’s been a while, but the last I checked crypto-driven GPU scarcity made it much harder and more expensive to build a gaming PC than to buy premade.

I bought a Chromebook for $92 delivered to my door this winter. It’s definitely not the case there.

Realistically, you can't even really build your own laptop from scratch in the sense of buying a case, motherboard, etc. OK--maybe someone offers this--but it's hardly mainstream. And you can buy a Dell desktop for ~$800 and I'm sure I could find cheaper deals elsewhere. No, it's not an optimized gaming machine but most people don't have any use for that.

There is value in building with a combination of new and used parts. Why do you need a new case each time you build a new PC. There may be cosmetic benefits, but an old case will rarely affect anything else. Likewise, I have a couple if bike frames laying around. I could pretty much build a bike to my spec and not bother with the expense of a frame.

Though bikes are another good example of something with a lot of standardization/interchangeability of parts. Most mechanical/electronic things around the house aren't like that to the same degree.

ADDED: I suspect people who say that it's cheaper to build your own PC are mostly thinking in terms of it can certainly be cheaper to upgrade a DIY system than to buy a new one off the shelf.

There are a couple of scenarios where you can possibly build cheaper. Consider when the purchaser is looking for certain components or to meet a particular specification. In those situations they would have to pay someone to build a custom system or pay the vendor to customize the system (in those cases the vendor frequently charges a premium).

Of course the big question is, why aren't household devices/appliances using standardized and interchangeable components? In many cases, it could probably be done. I suspect the main differentiator is consumer expectation. A significant number of consumers of bikes and desktop computers will repair, upgrade, and accessorize their purchase. Very few people will expect to do the same with their blender or television.

It's still completely true, except for the GPU shortage.