I don't think it's about whether they can afford it, I suspect they want to sell gliders in future (like they were going to with the concert one) and if we had too many free ones no one would buy them.

They likely wouldn't even need to hire more staff to make more gliders. Not exactly sure how Genshin operates but from what I've seen it's really common for 3D games to not have enough work for a full time 2D artist, so they either hire 3D artists who can also do 2D and have them handle UI, or they have one or two dedicated 2D artists who are put on random marketing content for the 80% of the time new UI/icons/whatever isn't needed. I get the feeling the latter is what Genshin does and the reason we have so many name-cards and web events to begin with. If that's the case then you could just chuck one of those people on gliders instead and nothing of value would be lost.