Why do many GA aircraft not have a "Both" fuel selector?
A quote from the book of regulations, Chapter 14, part 23, verse 951(b):


(b) Each fuel system must be arranged so that—
    (1) No fuel pump can draw fuel from more than one tank at a time; or
    (2) There are means to prevent introducing air into the system.



On a high wing aircraft satisfying #2 on that list is pretty easy: If you connect both tanks to the "top" of the fuel selector as in a Cessna 172 or similar then as long as you have liquid in at least one tank gravity will ensure there will be liquid at the top of the fuel selector to feed through into the engine:
 

The design of the system provides a passive "means to prevent introducing air" because gravity is always trying to pull fuel downward out of the wings and to the fuel selector.



On a low-wing aircraft like the Piper in your picture we don't have gravity to help us: There's no natural "head pressure" at the fuel selector (or at least much less than in a high-wing with the tanks mounted well above the valve), so we're relying on the fuel pump to draw fuel through the system.
This means we can no longer guarantee item #2 above: If the fuel selector were in the "Both" position and one tank ran dry the fuel pump is going to happily try to suck air through the line from the empty tank, and that could cause the engine to quit.  

It's possible to guarantee item #2 in a low-wing fuel system using float valves that seal the tank outlets when the fuel is below a certain level - an active means to prevent introducing air into the system - but that adds complexity and weight.
Since the aircraft already needs to have a fuel selector valve anyway it's easier to just design the valve in such a way to satisfy item #1 on the list: Make sure the fuel pump can only draw from one tank at a time - i.e. "Don't provide a "Both" position on a low-wing aircraft."