most of our musical experience has been with viola so while we can probably still fluently read alto clef sheet music and translate that to fingerings if we had a moment to remind ourselves of the reference points, we can't actually read treble or bass clef very fluently at all and also translating that to positions on a keyboard is very different from fingerings on a viola, and less intuitive I think
In other words: music is hard and musical ability on one instrument doesn't translate over to another instrument very easily
especially when that one instrument uses the rare-as-hens-teeth alto clef