What is an airport going to offer that an ordinary router doesn’t?
What Apple largely built its reputation on: a seamless user experience.
I used a few different routers before getting my first AirPort Extreme. The AirPort was easily the most stable router I'd ever used. It lacked some of the custom reporting features I had enjoyed using on my other routers, but that didn't really matter. I bought the latest AirPort Extreme (version 6) as an upgrade at one point, relegating my older version 5 as a range extender, and it was so easy to do. Setup could even be accomplished through the AirPort app on my iPhone. I connected a USB printer and a USB hard drive for Time Machine, and it all worked flawlessly with all of my Macs and iOS devices.
Unfortunately, for reasons I'm still not certain of, my AirPort Extreme 6 stopped functioning after a few years. Apple had already announced that they were stopping production by then. I could have bought another, as they were still available at that time, but I figured I'd best bite the bullet and get a high-end router from a different company. So I went with Synology, which people were comparing favorably to AirPort Extremes.
I'll keep a long story short: I had to fiddle with the settings to get networked Time Machine functionality to work reliably. It works fine now - I'm guessing it probably needed a software update from Synology, too. But my USB printer that had worked flawlessly as a network printer with my AirPort Extremes? I could never get that one working with the Synology. I ended up having to buy a "true" network printer, and even that didn't print properly without tweaking some software and driver settings. And this wasn't some fancy, multifunction printer unit; it was an HP LaserJet.
This isn't a knock on Synology. Unless Apple gets back into the router field, I intend to continue with their networking equipment. But it highlights pretty starkly what Apple was doing for their customers. It was a strength of the Apple ecosystem, that having a Mac or an iOS device meant you could easily set up your router, and that keeping with Apple's devices meant good quality and a flawless user experience for those Macs and iOS devices. It was also another brick in the "walled garden" that keeps people inside of Apple's ecosystem. Sure, you could still use your router with a Windows or Android device, but it wouldn't be quite as seamless. Routers from other brands don't go out of their way to make the experience better on Apple's devices.
Apple has been working on HomeKit for some time now, and it has the potential to be really nice. But let's face it: if I struggled to set up a networked printer in the year 2019, is a "smart home" really going to work as flawlessly as we're all expecting? If Apple were in control of more aspects of that connectivity - that is, the router - then maybe they could ensure a smoother experience, similar to how easy things were when the AirPort Extreme was the hub of my network. As it is, I anticipate things will be pretty kludgy whenever I have the chance to try them out.
(For what it's worth, I've heard that Apple also used to make their own printers, but stopped once printers became fairly straightforward... which is fair. But routers and connectivity have a lot more complexity, and it's still an area where I think Apple left prematurely.)