Revenue. They aren't free to build and it probably cost a great deal to develop. Is it a success if you sell $700m USD but are still $4 billion* in the hole?
* Made up number.
* Made up number.
Others have mentioned as well that it seems like this platform might not strictly be a bet on VR. It might also be a proving ground for vision systems and realtime processing capabilities. If that's the case, the information and development advances they can make with this experience might allow them to execute on bigger plans. That could be a car for example. I'm not sure what I think of that, but I don't doubt the product may have been partially justified because it allows shared progress with bigger picture plans. That would make going in the hole okay in a sense, because you'd have to go in the hole either way. At least this way you get a bit back and prove out technologies in the wild.
The company I work for expects each individual R&D effort to pay off and track that effort under a myriad of overhead/IRAD/misc charge numbers. Companies like Apple, Alphabet, and Microsoft just shovel money into that fire.
I obviously don't know what their costs and financial and sales plans are wrt. the Vision Pro but I think it is fair to assume that the pricing is set for a high margin within that plan.
Now, considering the high price and seemingly niche target, personally I am impressed that they already sold 200k units.