Tom:
There are many items from a very wide variety of LS manufacturers that are fully compatible:
First of all there is track gauge. All 45mm, to the chagrin of some!
Then there are the scales: Consider 1:32 (the ‘accurate scale / gauge combination for standard gauge on 45mm gauge track). It is supported by at least a couple of major manufacturers with several new kit and parts suppliers. As more scale modelers (as opposed to toy train operators) enter the fray, this scale will probably pick up more support. Then take 1:29: An incorrect scale / gauge combination, but with strong support from numerous manufacturers, including Aristocraft, USA Trains, American Mainline, and several others. 1:20 scale is the correct scale / gauge combination for 3’ narrow gauge models running on 45mm track and has good support from Bachmann, Accucraft and lots of kit manufacturers. Even the more obscure scales garner good support.
Most LS modelers don’t run a 1:32 scale SD9 with a 1:13.7 scale 2’ gauge Porter. The items within each scale/ gauge combination do have a high degree of compatibility, but there is certainly an apparent lack of compatibility if you are naming everything that runs on 45mm track “G scale!”
The items you mention are certainly open to discussion, but I have to disagree with your conclusion. First of all, couplers: Those Aristocraft couplers are designed for fairly modern 1:29 scale trains. The Bachmann couplers are designed for their 1:22.5 scale line of cars from an earlier era. There is a significant difference in both the era and scale of those cars. It would only be natural that they wouldn’t be fully compatible, regardless of looks. And yes, Bachmann does use a lower height than most, but that is also part of their narrow gauge focus. HJ has suggested the coupler solution (Kadee) many folks use, but even Kadee has two choices of coupler size. Again, this choice is primarily driven by the modeler / user’s chosen scale, prototype and era.
Next, the track: The Lionel & Bachmann tracks you mentioned are really considered fringe items by most LS model RRers. I don’t think Lionel track is currently produced, and Bachmann track is only good for indoor use. The track available from both these manufacturers is limited to the sharpest radii and can only be used for the smallest locomotives and rolling stock. Many of us would say there is no reason to “. . . make, say, Lionel switches work with Bachmann track . . .”!
While compatibility might not exist in what you term “. . . G scale . . .”, it does exist within the individual scales and eras you are lumping together. Just as the 0-27 & 0 scale Lionel trains were incompatible with Marx S scale trains, there are limitations in what is termed “Large Scale.”
Hope this helps in understanding why lots of LS things don’t work with other LS things!
Happy RRing,
Jerry