Not just built in, but intentionally difficult to remove. Apple uses proprietary screws to keep people from using tools they already have to open the things, for a start. Why should you have to buy special tools to get into your own computer just because Apple wants to keep you out?
Ya know, I don't think Apple gets this whole "selling" thing... when you buy a computer from them, it stops being their property and starts being the property of the buyer. It's not like what the courts have allowed vendors to get away with claiming for software, where you buy it but don't ever actually own anything. The customer still buys and owns hardware, in keeping with the same concept of "buying" that has been understood for thousands of years of human history.
Apple's wishes for the device cease to matter the moment the thing becomes the property of the customer. If he wants to open it, it's his right to open it. If he wants to repair it, or to try to repair it, or to hire someone else to repair it, he has the right to do that too, and performing such a repair should not be thwarted by the company that no longer has any ownership of that item. A simple addition of a jumper wire to bypass a ruined PCB trace (instead of the Apple-approved method of replacing the entire motherboard, including the CPU, RAM, GPU, and SSD, which are all soldered on, at which point the "genius" tells the owner it would be better just to buy a new one) doesn't turn a Mac into a PC and make running MacOS on an actual Mac into piracy (since MacOS is only allowed on actual Macs, and a Mac repaired by !Apple makes it into !Mac, in what passes for logic in Cupertino). That's not to say that certain actions taken by the new owner of the machine won't invalidate the warranty, but again, that is the choice left to the owner of the machine.