Personalized recommendations is not the same as personalized ads. You get recommendations depending on games you have played, but the ads seem to be general for everyone.
Its a lot of work to customise the store for every bit of hardware. The same way that the store suggests pc games if you browse on Mac. Remember, the store Is the same for everyone
It's literally one line of code to check the user agent. It's probably not much more than that to set a value like owns_deck=True in the session based on the user profile.
Honestly, I doubt it has anything to do with the complexity. More likely it's laziness or maybe it just never occurred to them.
Yeah it's one line of code to check the user agent but how many lines of code is it to customise each visit to the store front with only content as curated by the device the user is using. Even just user based curation is a lot of work and hard yet they do it because it draws sales. Why the hell would they spend all this work to not show you things. The steamdeck banner is probably considered as part of any promotion internally to the system so not showing it would require filtering every single promotion based on user agent and user logged in.
I don't have the time to watch it all, but I remember that the Steamworks Development channel on YouTube had recently-ish released this video about how games get surfaced to players and it also talks about what parts of the store are personalized and which aren't.
In the video I can only see the small ad on the left side about the Steam Deck. You're talking about the big banner ad that appears somewhere inbetween the sections? I can only guess they put it there for everyone, or maybe just every region that can purchase a Deck, for simplicity.