I can tell you that it's very easy to add map pings and notes to a game, and it's relatively resource-free, as long as there is an infrastructure to support it. I can't imagine why there aren't more developers doing this, I guess they just don't think it's necessary.

Its like resturaunts that won't let you modify orders or don't salt and pepper on the table, because the chef thinks that the only proper way to eat his food is as he's concieved it.
These developers surely believe they've given you enough information that you should be able to solve the quest or find a location, and that you don't need any more, or they want to punish you for not paying attention.

I know it's easy, but there's probably a long wish list and it's just not at the top. Other features like saving / reloading characters at creation, modifying the portrait from the game, and so on, must be in higher demand. The idea of having to write down notes may even scare away 99% of players, who'd rather have the thinking done for them.

I don't believe the developers explicitly wish not to have such a feature, it's a long game and it's not so much a problem of attention than a difficulty to remember all the details, especially if you can't play it in one weekend. What they probably don't want to have is a journal that literally says "go to <location>, talk to <NPC>", but that's different. If the players jot down a few notes, they processed the information themselves.

Playing underrail at the moment. So glad they gave us the option to add notes on the maps.