I don’t understand why they are not reintroducing the Apple Xserve?

Target audience - B2B. There are multiple videos of Steve Jobs saying that he hates B2B, because the people using the devices are not the ones making the purchasing decision. It is pretty much against Apples DNA, and all their B2B they have today is a means for them to sell more B2C.

Because they killed the market, no one would now buy a macOS server, when Linux distributions, and to a lesser extent FreeBSD, own the server room.

They would even sell less than Windows Server licenses.

By the way, they are down the same path with the workstation market, now that they only top level answer is the Mac Studio.

Workstation market wants flexible towers that they can customise to their own liking and special use cases.

The main reason Swift exists for Linux, is that app developers need to have servers somewhere, and if they want to share Swift code with the backend, well it isn't going to be on macOS Server.

My guess is the OS. People who want a server often enough want to choose the OS, Apple wants to supply the OS and the hardware together so they're not blamed every time the two turn out to be incompatible, as happened the other way round in the 90s.

whos going to buy one? You cant trust them not to kill it within a few years and cease all software updates AND make it impossible to install a different OS to keep it going. Until they stop being dicks about what you can do with the hardware you own it's a non starter.