I've been trying to rack my brain around this and asked a few CFI's who couldn't give me more than "because that is how it was done", but a good number of GA aircraft save for Cessna and maybe a few others only have a Left/Right fuel selector:
Image Credit: Make Time For Flying Blogspot
To me, as an engineer, it makes great sense to have a Left/Right/Both selector such as found in most Cessna's. This lets the pilot focus on flying the aircraft rather than having to switch tanks every 30 minutes to balance fuel out. A Left/Right/Both selector isn't sigificantly more complicated than a Left/Right selector, so that leaves me wondering...
Why do GA manufacturers opt to use just a Left/Right selector valve given that a majority of GA accidents are fuel related and off-airport landings with fuel in a tank is still relatively common? Is there a safety issue I'm not seeing?
I understand that even with a "Both" selection, fuel may not drain from both tanks equally, but you can always switch to the fuller tank for a little while then back to both in that case.