I'm not sure whose decision this is, or why, but BZ tickets are not available on TOD, which presents a big problem for websites. Without that, you can't really offer them online, because it would not go down well to customers to insist on charging a postage fee and how could such websites explain that?
See:
Boundary Zone tickets are not permitted to be issued as "Ticket on Departure" (ToD) tickets, so even if the website could sell them they would need to post them out...
... they are not compatible with the web retailing model of requiring a journey to be generated to establish validity.
There is no real issue with an origin/destination of a zonal location; this is already the case with "Zone U
x London" and "London Zones
x" tickets which are available online.
Most websites do require an itinerary to be generated first, but it should then be possible to determine which tickets are valid for that itinerary.
For example if you were to search for Amersham to Stansted on Trainsplit, you would be offered Amersham to Harrow on the Hill plus Zone U1245 - Stansted singles. There is no good reason why Trainsplit should not be allowed to offer Amersham to Boundary Zone 6 single, plus Stansted - London Zones 1-6 Travelcard instead, if that was cheaper (I've not done a comparison of the fares; this is for illustrative purposes only)