It frustrates pilots as well. However, the sticking point is not the mostly recreational usage. Most of the pure recreational fleet could burn an unleaded version of the current 100LL. That wouldn't "solve" the problem though.

I'd love nothing more than to see a safe, usable, unleaded aviation fuel emerge from the morass that is the FAA approval process.

I want an unleaded replacement fuel as much for the sustainability of the necessary part that general aviation plays in the economy as for the ecological issues. There is only one chemical plant in the world that makes TEL today. We are quite possibly one significant industrial accident away from having the fuel supply disrupted.

I believe 9 candidate unleaded fuels were submitted to 1 testing avenue, though I think that the most promising fuel, G100UL, was not submitted to that testing process. Presumably GAMI (the creator of the G100UL) has some alternative certification process that they're intending. I do know that the Shell, Swift, and GAMI fuels have flown a fair amount in experimental airplanes already. That's a technical categorization. The airplane itself was probably mechanically indistinguishable from the certified airplane, but was placed into the experimental category specifically in order to test its flight characteristics with the new fuel. A certified airplane cannot legally fly on non-certified fuel (or with non-certified radios, autopilot, non-certified diesel engine, etc). Convert that same certified airplane to experimental, put the required placards on it, and you can fly it with the non-certified fuel.

To address the "why don't you just burn car gas?" question: the high-compression and/or turbo-charged engines typical in the working airplanes require an octane rating higher than currently available car gas or unleaded aviation fuel. Most (some estimates are over 80%), of the low and mid-performance piston engined airplanes could burn 94UL (the current ASTM D910 standard 100LL just without the lead added at the end of the process).

The relatively low percentage of the fleet which cannot burn 94UL is unfortunately the portion of the fleet that flies the most hours and burns the most fuel. Estimates are that this 20-25% of the fleet burns 60% or more of the fuel. These are the freight haulers, charter operators, CapeAir and other island hoppers, Alaska mail/grocery/fish/charter ops, firefighting water bombers, small business aircraft, and high-performance family airplanes. (That last category doesn't burn near as much as the planes that fly everyday on working missions, of course.)

I sympathize with the FAA a lot, as the standard (ASTM D910) for the fuel they're looking to replace is a chemical makeup standard and not a performance standard. The standard literally dictates that it contains a certain amount of tetraethyl lead (TEL), rather than dictating certain performance characteristics. Therefore, it's just as impossible to create a "standards conforming fuel without lead" as it would be to create "water without the wetness".

So, why can't we just have airports start selling the 94UL at least to those planes that can use it?

Two reasons:
1. Aviation fuel is already a low-volume fuel, and requires special handling all the way from refinery to dispensing station. If you believe that 94UL sales would only be 1/3rd of the current fuel sales, you now need a duplicative system of trucks, tanks, and dispensers for a volume of sales that's 1/3rd of the current, already low volume. The economics of that are daunting to say the least, on a per-gallon-dispensed basis. Airplanes are, by definition, mobile. If 94UL costs more than 100LL due to low-volume of sales, airplanes will fly to a field that sells 100LL. You can't readily (practically) ban the sales of 100LL to airplanes that could use 94UL, because you don't want airplanes taking off from airports that only sell 100LL with inadequate fuel and crashing from fuel exhaustion.

2. If an airport were to decide to take their existing single avgas storage tank and switch it over to 94UL, they'd be turning away 2/3rds of their business. If they decide to keep the single dispenser selling 100LL, they can sell to everyone. Again, the economics are compelling in favor of a single fuel solution for all piston airplanes.

I happen to believe in G100UL, but I'm not sure I see the clear path to getting any of them approved.