Adhesive is needed because there can’t be any pressure against the underside of the display module, and the battery can’t be allowed to move even a slight amount during drop impacts.
They're thicker to account for a solid plastics barrier between the battery compartment and the display backside to protect the display from the user during battery replacement. Glueing the battery to the phone backplate allows the manufacturer to skip the .5mm of plastic.
I realize if this logic was applied to everything the phone could double in thickness, but for a consumable part like a battery, it seems worth the sacrifice to make the repair trivial for the average user armed with nothing but a small screw driver.
People not, but marketing people do.
There isn’t really a marketing strategy of “nobody values this but we will fight to the death to get the product designed that way so we can spend a ton of money convincing consumers to want something they don’t”.
Or, let me rephrase that, there is no sustainable marketing strategy like that. People do try (see: New Coke, Humane AI pin).
[1] https://collections.museumsvictoria.com.au/articles/1538
[2] https://arstechnica.com/gadgets/2024/03/intel-revives-the-mh...
(Manufacturers probably also like phones that have to be replaced in a few years due to battery age.)
But simultaneously, I don't know if that many people would consider it to be selling point either nowadays. Battery capacities don't seem to shrink as quickly anymore, such that the phone probably gets damaged/stops receiving updates/is replaced anyway by the time the battery would need swapping.
Although perhaps that'll change now that the changes between even 2-3 year models are getting pretty small and software support periods are increasing.
I will rather pay the extra 50€ in case I need to change the battery in a few years.