The reason would most likely be low demand.
Edit: Turns out, sash windows are more commonly found in the UK (compared to other European countries), but still not as common as in the US. So, UK = not as hot (so far), thus still probably not worth it (yet) as a market.
But you should still be able to get two tubes fitted into any kind of window with the right seals. If you were really up for renovations you could get closeable exhaust holes punched through your brick or something maybe.
And yes, there are options for tubes/ducts for the more common window types. Like tilt-and-turn windows, horizontally sliding or all the other kinds of inward or outward opening windows - but most of them are the ducted portable units the original comment was speaking of, which aren't great. There are also some better portable split units, but those are pricier and the install is not as easy. (They're great though.)
I also sometimes miss being able to stick my head out the window but that's a tradeoff about screens and in the end of the day I prefer not to have to worry about bugs.
Best of the best is about 15-16 SEER
That's entry level central HVAC efficiency
Minisplits are far higher, 20+
That’s way high: You can buy 2 for that much if you want a big name brand that provides support for self-installs. 3-4 of them for $5k if you don’t care about that…
Source: I’ve done it
That's completely false. They work just fine despite not being terribly efficient at least provided you install them correctly (but that caveat naturally applies to any window unit).
In fact despite the low efficiency using only one in a single room is likely far cheaper than cooling the entire house. It's the same principle as an electric space heater versus a whole home heat pump.
Of course running a minisplit only in the one room would be substantially better but for a 1 kW unit the difference is less than $1 per day (unless you're subject to the California electric grid I guess).
Since these one tube systems just push hot air out, you are creating low pressure in your room and you will suck the air in from somewhere. Often times simply from outside from the leaky windows.
I don't understand why it's hard to buy systems with two hoses that would keep the pressure in the room.
As to the negative pressure, personally I just leave the door to the room cracked open very slightly. But if you have central heat ducts air will get sucked in through that no problem.
In system where the pressure would be equalized the leaks would be much smaller. For some reason it's impossible to buy these type units in EU.