Why do pilots need a medical

I think the "loigical" reason for non-commercial medicals is to protect
third parties (i. e. those on the ground) from our actions. What's
missed though is how rarely ground injuries or deaths are occuring from
single engine aircraft crashes. How many were there last year?

I think the need for medicals should be related to the amount of fuel
on board.

An alternative is I think the pilot community should demand biennial
medicals for all drivers licenses. Particularily given the number of
third party casualties involved. After all who can be against safety?