Wendigo Have you considered that that is because, as doublevr said, the limited market for that type of content is already oversaturated, and NAVR/MiLF are failing to make money on it?
NAVR started to bleed members when they pushed their pricing structure shenanigans some years ago. Without a doubt they lost countless subs on that and eventually had to pull it back but the damage was kind of done. After that it's been VR free fall and a turn towards 'not shooting milfs' in VR and focus on regular pornstars obviously hasn't turned the ship around in regards to VR as they have kept reducing weekly updates.
On top of that NAVR has refused to fix thr issues with their setup of a blurry lens. Not to mention they insist on using cockzillas in VR which arguably many VR viewers find unappealing. In other words their VR issues most likely lies elsewhere than milfs not being popular in VR. NA after all build their entire brand on milfs.
The fact that MilfVR can keep updating with sterile autopilot rinse and repeat scenes and yet still exist, just speaks to me of a lack of serious competition rather than a saturated market.
The more likelihood imo for the lack of milfs was what one person involved in the industry said a few years back on reddit; high quality milf talents are either under exclusive contract or rather expensive. Furthermore the most popular milfs is in the process of or were retired completely. 'New' milfs to replace them has been in short supply as many of those popular pornstar that reached the right age simply jumped on the Onlyfans, Loyalfans, etc. wagon and won't shoot b/g scenes or again locked in exclusive contracts or very expensive.
So it's far easier and more importantly cheaper to pump out scenes from a sea of either cheap desperate teens that are trying to make a name for themselves or a high quantity of more popular names that hasn't quite found other pastures yet.
This was kind of backed by VirtualTaboo that stated a few years ago that milfs/matures indeed were in high asking demand but it got increasingly difficult and expensive to hire them. This answer came ironically from a discussion of why they seemed to shift focus towards teens instead of milfs.
My point with all of this is that the reason for the lack of milfs/matures in VR are likely found elsewhere than a lack of demand. All of the above could be wrong of course. I can't verify anything as I'm not an expert. I just pick up what I read here and there by insiders and I've also written with high quality talent on Onlyfans and they kind of confirm the above.
Furthermore I do find it hard to believe that a category that has continuously been represented in the top searches and watch list on free pornsites for over a decade worldwide now, somehow is not in demand in VR. Could be of course, I just doubt it.