> Why can't people walk one flight of stairs instead of three, two up and one down?
Presumably because the management of at least one of the two systems thinks that the initial and ongoing cost (and therefore, ceteris paribus, fare) increase of either system integration or additional fare gate infrastructure (the latter of which may not be practical due to space constraints on the platform level) isn't worth the convenience increase, at least compared to other improvements they could make to their systems at the same cost.
Building and maintaining more entry/exit gates doesn't have an upfront and ongoing cost? Integrating Muni and BARTs entry, exit, and ticketing systems wouldn't have upfront and ongoing costs?
I suspect an solicitation of bids for either of those would not have any $0 bids.
Presumably because the management of at least one of the two systems thinks that the initial and ongoing cost (and therefore, ceteris paribus, fare) increase of either system integration or additional fare gate infrastructure (the latter of which may not be practical due to space constraints on the platform level) isn't worth the convenience increase, at least compared to other improvements they could make to their systems at the same cost.