One reason I could see is to keep perceived price level for the frequent flyers. If they don't see best deals they won't expect those. And the hardcore deal hunters are not loyal to you or your brand.
Giving frequent flyers very good deals make them expect them, thus not be so willing to pay "regular" price.
Virtual interlining is really dangerous model for them and their brand. It basically means that they would be reliant on someone else without any guarantees in case there is delays or problems.
And with different alliances they already have fully functional interlining model. So trying to extend outside this is not beneficial for them.
Well someone at the airline does need to crunch the numbers to figure out at what $X of spend the risk becomes sensible to take on.
Those flyers don’t care. Most frequent flyers are using other people’s money (business travelers) or are not price conscience.
We don’t even compare prices for domestic flights and we fly a lot on our own dime (15x-20x+ a year since mid 2021). We instinctively just book Delta where we have status and lounge access.
They do care when there is no valid routing from point A to point B via official codesharing/interlining/alliance partner airlines on a given day, forcing an overnight layover, or virtual interlining.
Usually this happens when flying long haul business class from smaller airports to another small airport on a different continent.
They could easily position it as a reward for loyalty for FFs who spent more than $X in the last calendar year with them.
The loyalty reward in this case would be treating virtual interlining tickets as if they were an actual interlined ticket.