Most AAA parties don't release to the switch because the architecture is a lot different, no simple port.
Modern engines like Unreal and Unity, by far the most used ones by the 3rd parties, support both architectures with no problems. If the game isn't too ambitious in the technical side, is small enough and doesn't require to tune it down too much (native resolution, fps, textures, amount of stuff on screen, etc) due to lack of horsepower there's no technical problem at all to port them to Switch. With Unreal or Unity it's easy and fast.
The thing is that many games are in a somewhat middle ground where they could have a Switch port but needing a ton of optimizations, tweaks and downgrades to keep it with a decent quality. Maybe needing too much work and budget compared with the limited sales that multiplatform games have on Switch. And in many cases aren't sure if they'll be able to achieve it or not, so they end avoiding the Switch port.
My point on PSVR is Sony could be dipping their to in the water in something that could do a lot more sales, instead they are focused on something that at this point has a very low ceiling.
We don't know the exact sales of PSVR1, we only know it sold over 5M and that Sony was happy with it. With Vita instead we know it sold around 12M and that Sony wasn't happy with it because PSP sold around 80M.
Nintendo became way more powerful in portables because merged there their teams and catalog from home consoles. And now there's also the PC handhelds, and the mobile market keeps growing, so there's less market for non Nintendo portable consoles than before. And well, Switch got benefited from having no competition for a while and getting the role of "portable 3rd party indies & AAAish machine" between era of Vita and the PC handhelds. I think the PC handhelds will eat a huge chunk of that Switch market and the Switch successor will be less successful than Switch.
As happens with cloud gaming, VR instead is an emerging, growing market with a lot of potential that is making its first baby steps. Sony invests in VR not to sell a ton of units now, but instead as R&D and innovation testing the waters of that emerging market building related technology, expertise, catalog and knowledge, positioning themselves in a good position and also to have an additional unique selling point that adds an extra point to their consoles: PS are the only only ones with high-end VR and the ones with the biggest cloud gaming catalog.
There is plenty of room for competition, the history of game consoles tells you this over and over and over. Should MS not have launched the Xbox since nobody would buy it?
So the massive sales of the switch make you think nobody would buy them? Makes no sense.
If you want to play the MS games or most 3rd party games on a portable, plus basically all games from past gaming devices via emulation, you can do it on a Steamdeck, you don't need a MS portable console.
When Xbox debuted in home consoles there was market for them, specially when Sega quitted or when Nintendo decided to focus more on portables and casual/family friendly stuff than in hardcore games with high-end visuals. Xbox consoles don't sell a dozen millions units like Vita.
Also, MS until now struggled to have a constant and frequent good amount of big first party releases, but with all the recent acquisitions and new internal teams created seems that in a year or two they'll start having a good enough output comparable to Nintendo and Sony, if not bigger. If they would have to cover another device they'd struggle again, as happened to Sony with Vita.
A Sony or MS portable would have their own exclusives, Sony could literally launch with every major franchise in their catalog, and they don't need a steam catalog at all. 3rd party developers would jump on board due to the super cheap and easy ports.
They can both compete just fine.
I'm a PSP and Vita super fan, but Sony wasn't capable to provide Vita a good enough amount of exclusives to make it successful, and AAA games now take more resources plus also have to cover VR, pc ports and mobile games. Sony doesn't have enough resources to properly give enough exclusives to a portable. MS had issues to provide a good enough amount of great exclusives to their home consoles, so would struggle even more if also had to make exclusives for a portable.
Sony and MS will support Steamdeck with their PC titles, and will also bring their console games to mobile via their cloud gaming service (Sony should re-release theirs soon, pretty likely in 2023), plus a few native mobile ports. They may also release dedicated gamepads or adaptors to play consoleish games on mobile. That's all.
As said numerous times the only reason MS should not be allowed to buy Activision is if the law is being broken. Hurting Sony's market share is not a reason to block the deal.
In fact, looking at the market numbers as MS says it wouldn't almost affect Sony's market share and Sony would continue as the market leader in consoles and game subs, and also the top 2 in the overall gaming market.