Pervert! Let the girls breathe!
Its not breathing that's the problem... its the being stepped on, squished, pinched in between stuff, clawed... with 10 cats and two climbing children that see Mom as a jungle gym... well, things get hurt. Best they stay in their casing.
I believe @JustGeorge is quite ample, being let loose to breathe can be rather uncomfortable, uncontrollable, cumbersome and as George has mentioned, tend to get in the way of family life. It's all a matter of convenience