Unless you are talking about panel sizes of 60" and more, that hasn't been true for years. There is a premium compared to LCDs, but the prices are far from "prohibitive".
Just because an OLED TV has a HDMI port in it and you could plug a PC into it doesn't make it a good computer monitor.
Can you name a 'far from prohibitively' priced OLED computer monitor please?
To save you some time I'll name two OLED monitors for you. Dell UltraSharp UP3017UQ - on launch it cost $3499 and Dell don't seem to sell it any more. Do you want to see an iMac (not even a Pro) start at $5k with i5 and a hard drive in it?
Want one that you can buy now? Alienware AW5520QF - on the Dell UK store it's on sale for £2799. Yeah it's 55 inch and marketed for gamers, but where is the HDR mode?
Too big? OK, Asus PQ22U, a 4k 21.5" display, right? Lovely. Factory calibrated, HDR, 99% of DCI-P3 gamut, and very close to professional grade displays if you only looked at sections of the screen. But the brightness isn't that great on this one - however, crank up the brightness and the burn in chance increases. How much? $4k - or almost £3,900 on Amazon UK. Yeah, I don't see Apple putting that panel into the iMac 21.5"...
I've seen smaller laptop sized panels but read the reviews - tell me how long till you reach the comments about burn-in.
You'll have to look at the definition of prohibitive from a consumer point of view. Imagine the outcry if an OLED iMac screen burns in after 3 years when the rest of the computer could do service for 10? And they've had to pay hundreds (thousands) of extra dollars for the screen? Look at the number of OLED monitors on the market and the cost of them.
Apple aim for a price point and obviously decided that OLED fits the profile for phones and watches and would be unwilling to apply that premium to a computer. Imagine the utter outcry from certain Macrumors denizens if the next iMac came at a $2500 base cost because of an OLED screen despite Apple using power.
Prices are coming down, LCD panels are becoming commodity and it's easy to see why Apple are getting off that particular horse and getting on a more valuable quality proposition - and that proposition is Mini LED and not OLED for iMacs.
You did notice that Apple uses OLEDs in their top-of-the-line iPhones and Apple Watches, didn't you?
Perhaps I needed to clarify the statement to mean monitors for Macs and MacBooks. And OLED may be an acceptable stepping stone for phones and watches that won't last a decade and aren't 'on' for hours - showing a static screen all day. That's the difference that makes OLED acceptable for phones and watches.
Note that iPads have stayed LCD and will be going direct to mini LED rather than ever getting OLED.
OLED colour reproduction may be superior but I doubt the people (like myself) who keep their Macs for the best part of a decade (or indeed the people who buy used machines) would be impressed with OLED panels after any appreciable length of time and used values would plummet even if the machine was in otherwise perfect condition.