Because it's the only Mac between a $500 Mini and a $3500 Mac Pro. A LOT of people would love to buy a "monitorless iMac".
Very much this. If Apple made a non-Pro non-mini just-plain "Mac", say a 6-8" cube, with no monitor, a desktop level CPU and slots for the GPU, RAM, and storage, they'd have to beat the resulting crowd of enthusiasts back with a stick. (Mass market might go for the iMac, but gamers, developers, enthusiasts, etc., would jump on a headless Mac.)
Apple's monitors have traditionally been really nice. An Apple-branded television would no doubt be gorgeous as well. But if you buy a TV that'll be great for
many years (my TV is going on 8 years old and I'm not in a huge hurry to replace it), with a built-in AppleTV that'll be great for, say,
two years, then very soon you'll have great screens with obsolete guts. Many people would not be happy replacing their otherwise excellent TV every year to just to get improved guts. They could make an Apple-branded television with some sort of slot for an upgradeable/replaceable AppleTV, but then that isn't far removed from the current situation with just using HDMI.
I'd venture a guess that, rather than slotting in an Apple-TV-on-a-card into an Apple television, they might simply go for some expanded communication channel between the screen and the AppleTV for better integration (so the AppleTV can always be in complete control right from boot time, and such). They could perhaps do this over the existing ethernet facilities present in HDMI, or by expanding the HDMI standard.
Alternatively, I could see Apple releasing what is essentially a very nice 50"/80" (or whatever size)
monitor, with no tuners, antenna, or cable inputs, and just one HDMI port (or some sort of improved HDMI port), and likely with a very nice built-in sound-bar equivalent, designed to be your new complete TV experience when combined with an AppleTV (with the $100-$200 ATV being replaced every few years while the $1000+ screen sticks around).
Imagine a wall-mounted Apple screen with six speakers at the four corners and top/bottom (arranged like the pockets on a pool table), similar to the four speakers an iPad Pro has, along with an optional subwoofer and surround speakers all talking to the monitor over WiFi (great sound for daily use, optional theater experience for movie night). They could also incorporate a camera and mic, for FaceTime/video conferencing, and the mic could work for Siri in the living room without the remote. These are all components you could put in a "monitor" and they'd be good-if-not-great for many years, while the control box, the AppleTV, can get replaced as technology improves.
