Because when you can hear the toy going when you are several rooms away, you know the kid is still there and still playing with it. So you can be in the kitchen doing housework or whatever and hear it over clattering dishes etc.
When the noise stops, you know they're up to something they shouldn't :) Nothing more worrying as a parent of young kids than silence!
This is an interesting problem. I'm not an audiologist, but am sure that any noise above 75dB is harmful to an _adult's_ hearing. If the case can be made that infants and toddlers hearing is more sensitive and should be accounted for, then by leaving the toys so loud we're choosing function over the potential well-being of the toddler's hearing.
When the noise stops, you know they're up to something they shouldn't :) Nothing more worrying as a parent of young kids than silence!