Tablet takes less effort to move and you don't have to physically move your body as much which means less strain. With a mouse you have a fair amount of kinetic energy to stop when you change directions.
You also have more precise control in your fingers from writing your whole life. Writing letters with a mouse is much harder than with a tablet, and that translates to osu.
And the biggest thing is Absolute tracking, which means the cursor will ALWAYS be in the same place on screen in relation to the pen, which builds much stronger muscle memory. Even to the point where you don't even really need a cursor anymore.
Your brain starts to get rid of the disconnect between hand and cursor and it starts to feel as natural as a touch screen. Like you know exactly where to move the pen to hit a circle regardless of a visual cursor on screen.
That just doesn't work with a mouse because mice have relative tracking. even if you never lift your mouse off the mouse pad, the mouse will drift around so you never really have true muscle memory for the relation between the cursor and position of the mouse.
Overall there is really no objective advantages to a mouse for osu. Other than being more convenient.