...here are some RPGs and and similar games from the last 15 years that had day/night cycles...
Most of those examples all had
cosmetic day/night cycles - NPCs didn't go home for the night to sleep, go to the nearest inn for lunch or spend most of the day at their workplace.
You're right to some degree. But even a cosmetic day/night cycle adds to immersion and opportunities for roleplaying. Putting the player in a game world which is perpetually bathed in daylight is about as immersion breaking as you can get. And for those who like to play as thieves, well, no night is a serious issue.
There are, however, some fairly simple ways to add strategic depth and the illusion of NPC schedules to a day/night cycle without having to actually code it in completely. Closing shops at night by simply locking the doors. Making stealth more effective after dark. Making certain monsters appear only at night, and others only during the day. Etcetera.
