Because it's a MacBook; with batteries in it. The value is in the mobility. If you don't need the batteries and mobility, you could buy a Mac Mini, iMac or Mac Pro.
It's not uber-thin and light, no; this is a literal desktop replacement and weighs twelve pounds. It's not the machine I'd buy, because I don't need a GTX 1080 in a portable machine--but that machine is equivalent to Apple's best, actually-serious desktop while being cheaper and portable! It goes without saying that you can scale down to something that's a little more reasonable for a physical-effort-averse nerdy type and still get something that's very favorable compared to Apple's offerings, both in terms of perf and price.
Apple missed the memo: you really can have both, these days. Maybe they don't care about that audience anymore, and that's their prerogative, but it's why I'm probably bouncing back to Windows/Linux. (Dual-booting. Ew.)
It was released in October 2013, so more than 3 years.
As it stands, you can neither recycle it as an external display nor can you run Linux on it at full resolution.
Not really a "pro" complaint - just something to keep in mind if you plan to hand it on.
For some people. But you can also look at a laptop as being the ultimate "all-in-one" PC.
The only cable you need to plug in is the power cable. For a lot of people, that's more convenient than having to deal with a monitor, keyboard, mouse and desktop with the associated mess of cables and added desk space. That a laptop is portable and runs on a battery is gravy.