Because an infant's eardrums are very fragile. You shouldn't make noise around them at all. Unless you want to be personally responsible for them being partially deaf.
You also don't take an infant out in the sun, because their corneas are very fragile. The reason 60% of young people need glasses is because their dumbass retard parents took them to the beach.
Please Babies can sleep through anything, so you can make noise around them. As for taking them out in the sun you can take them out in the sun, so long you have a paraol for the buggy.