They aren't though - that's the problem with 14.1. Some balls are not in play because they don't go anywhere until you make it possible to make them go somehwere by clearing blockers or opening clusters. On this point, 14.1 can be excruciating. It's way too easy to bump balls and have no shot, bump balls and just make more clusters, etc.
Some aren't in play because you need them to close out the rack (i.e., they are the final pattern). Now, for sure, the opportunity to play for multiple balls is greater in 14.1 than in 9 ball, but as the rack goes on in straight pool, it get's more difficult as you close out your pattern and your margin of error in position gets increasingly tighter; whereas in 9 ball, the rack gets easier with much larger margins for error in position as you close out the rack.
To me, this is what makes 14.1 immensely enjoyable and bananas hard.
All IMO and respectfully.