Because you remember it better when you actually write it out instead of just using a keyboard. And you can draw diagrams with ease. Most styluses are inaccurate and one dimensional, and buying a phone with actual proper stylus support in both the display and stylus itself is expensive. You could buy a separate technical device just for note taking with proper stylus support and have it upload notes to the cloud so you can access it at all times, but that requires a constant internet connection and mobile data is expensive. And then you have to carry this seperate device with you in the same way you’d carry a much cheaper physical notepad anyways.