The ETA tells the average passenger what they want to know. What good does knowing the distance do you? If the ETA is bad the analysis should be improved. There's little point in encouraging the passenger to try to outsmart that analysis. The designer of the system has much more information to work with.
Sure it would be be nice to supply that information in some open API somewhere though.
Time makes a lot more sense than stops: for example, at Columbus Circle in NYC, a downtown A train that's one stop away is about 3.5 miles out. A downtown 1 train that's one stop away is less than a half mile out. Except if the downtown A train is running local, in which case it's then about a half mile out.
Time, on the other hand, can be meaningfully displayed to a rider no matter what information they already know. (It's also fairly accurate in practice in my experience.)
It's accurate as long as trains are moving. As soon as they are stalled, that time stops reflecting the real one. So showing the distance at least gives a more appropriate info, instead of showing "5 min" for 15 minutes (which is annoying).
Sure it would be be nice to supply that information in some open API somewhere though.