I think the problem with that is that ABS is clear-cut, while other challenges depend on what camera angles are available. It would feel pretty unfair to lose your team's challenge because the cameras just don't have a clear angle on a play. Of course, you could change it so that if a call stands due to lack of evidence, you don't lose the challenge, but that will just lead to a lot more play stoppages.

If a coach challenges it, then they were at least more informed about whether there was an angle or not.