Comment by inferiorhuman

  It's not like they could then buy cheaper off-the-shelf trains, since
  the trains still have to be designed and built for the particular system
  in other ways, i.e. controls, propulsion, and custom interiors.

They could but they've doubled down on American exceptionalism (or NIH if you prefer). For instance, Muni's Siemens trains (and the Boeing/Vertols and PCCs before them) are far closer to a standard train than BART's stuff. The real advantage is in cheaper propulsion and maintenance kit. BART runs a non-standard voltage and had a ton of trouble sourcing thyristors for their old fleet. Track maintenance vehicles also need to be custom built.

  BART doesn't share tracks with any other system, so there's no reason to
  do it for compatiblity's sake either.

BART doesn't share tracks because they can't. As a result they run three separate fleets of wholly incompatible vehicles for no good reason. eBART is a standard gauge diesel multiple unit system – because even with BART's typically inflated ridership estimates they couldn't make the expensive mainline kit work. That would've been a perfect opportunity to start the excruciatingly slow process of converting the mainline to more current standards.