why doesn't a PA28 have an option of using both fuel tanks at once..? so they will run out equally?
The answeer to this question lies in the design requirements which says:
Airspaces of tanks with interconnected outlets must be interconnected;
and;
There may be no undrainable points in any vent line where moisture can accumulate with the airplane in either the ground or level flight attitudes
In combination, these requirmeents mean that if you want to have a "Both" fuel selector option, the airspaces of the fuel tanks, (outboard ends of tanks on the PA28) must be interconnected, and the interconnecting vent line must not have an undrainable spot. This would mean that the vent line would have to come up out of the wing, and continue above the wing, right through the middle of the cabin, because of the dihedral of th wing making the outboard ends of the tanks higher than the inboard ends. .
This design requirement is the reason that you nearly never see aircraft with low wing fuel tanks with a "both" fuel selection available.
High wing aircraft usually have the fuel vent line cross up against the inside of the cabin roof, so as to meet this requirement, and allow a "both" selection. Though there are some high wing aircraft which do not have "both" (some Cessna 206 and 210).