well the main reason is you can't fast travel on a whim - you have to get to a signpost. At least for me.
This is one of the smartest design choices they made. Forcing you to travel a little before you jump leads to some great discoveries.
I stumbled upon
Letho
Same goes for monster encampents that house rewards such as the Witcher gear sets, or random encounters with higher level monsters.
Exploring this open world actually feels like an adventure.