"Apple Tax". It's dat logo.
Same reason that Samsung SSDs are overpriced.
I'd hardly say Samsung's SSDs are overpriced. Certain models might be, but it's more to do with paying a premium for bleeding edge tech. But plenty of the SSDs they make are priced more than competitively.
Also, that is pretty apples-to-oranges with OLD Apple hardware.
There's a lot of reasons Macs keep a high price. Mainly I'd say it has to do with supply/demand. Apple is just one company and they only make a small number of models, very little choice. A buyer has very little choice usually than to spend a pretty penny on a new Mac.
Therefore, trying to get an 'older' but still late enough model Mac to be usable- people can still charge a lot more for them, because the buyer won't have much choice.
Also the percieved value of Macs tend to be and stay higher for the owners. I have to say, it's justified a lot of times. There's not too many laptops of 2015 vintage I'd think much about, but I won't part with my 2015 15" Retina MacBook for anything less than the cost of an equivalent laptop today.
And that's going to be pretty pricey, as in 'not worth it' for either party. So even finding someone willing to part with their older Macs is often dificult, let alone going to be a steal.