The reasons for them to do non-PC releases of library games, however, is the same as Portal Companion Collection: it'd be easy, it would be a job which would generate some money, and it would be a nice moment for whichever of these franchises they chose to pull out of the vault. (Valve doesn't even need to do the ports, there are lots of Source-familiar developers who would be happy for the contract these days.) They probably won't bother, but weirdly, every once in a great while, they do or let somebody do it.
I don’t think you’re getting it; Valve is totally uninterested to put in any effort themselves to release their games for non-PC platforms regardless of how easy the job is. The only way it is happening is if their partner, Nvidia is the one doing it. if Nvidia wants to port to PS5 or mobile, it’s unlikely Valve will say no. But like I said, no reason Nvidia wants to do that in the first place. For that matter, I don’t think Valve even care to the extend of looking for other developers to port their games considering they have Nvidia.
OK, now you're overthinking it. Valve didn't make Portal Collection to "promote" its almost decade-old Tegra chip. (NVIDIA as it is has not been generating any Tegra business in a long time outside of Nintendo and some car and robot projects, and although the T239 is promising comeback as a follow-up to the Switch's X1 if they can slim it down, there hasn't been a line of other customers eager to put Tegras in their hardware for years.) And they didn't make the Collection because Nintendo has a strong partnership with NVIDIA. (NVIDIA and its developer studio wouldn't be getting any laurels or insight from Nintendo by porting 10-year-old games to a mobile handheld either.)
I am not saying Valve is promoting the Tegra chip. I am saying Nvidia is the one who is making the Portal ports to maintain and improve the current partnership (of Tegra chip) with Nintendo using Valve’s games, and even if they are not promoting Tegra at this point of time, it serves to improve their chance to extend the partnership for Nintendo’s successor.
BTW, NVIDIA's strong partnership with Valve didn't get any Tegra chips into Steam Deck; those are AMD chips powering that portable... maybe Valve should strengthen its AMD partnership by getting somebody to make special versions of old games for that device* too?
For Valve they are in Nvidia’s partnership because its passive income without any work; Nvidia upgrade their classic games, Valve sit back and collect the money. Doesn’t mean Valve wants or have to return the favor though.
If NVIDIA wanted to use old Valve games to make Nintendo slightly happier, they could have just taken their existent Portal 1 and Half-Life 2 ports from NVIDIA Shield and put those on a card. (Portal 1 in Portal CC is mostly the Shield Android port from 2014, and shreds of the HL2 Android are left in the build of Portal CC Switch.) Instead, NVIDIA Lightspeed made a fresh Portal 2 port, complete with online support.
I am not sure why you will pair Portal and Half-Life 2 though except that it’s easier for Nvidia. 2 Portal games in a bundle seems to make more sense. That said I don’t think Half-Life bundle by Nvidia isn’t impossible, but far more likely for Switch over Xbox/Playstation.
Again, NVIDIA Lightspeed has things it could be doing besides porting a couple of Valve's many games to Nintendo hardware. Their RTX remastering projects have been press-worthy works (although now with RTX Remix allowing full AI-generated replacements of textures and models, NVIDIA is turning to even smaller and cheaper MOD groups; only four people are being used to oversee HL2 RTX Remix.) But for whatever reasons, this particular project lined up to make happen.
You’re right that Nvidia have other things they can do too. But I don’t see the need to have any specific reason here for anything like choosing to release a Portal bundle for Switch over RTX remastering some games for some other partners. It’s not like porting Portals to Switch is stopping them from their other projects.