Or, less cynically, because no one uses 'gals'. There is no female analog to 'guys'. Males under age 18 -> boys. Males 18-35 -> guys. Males over 35 -> men.
For female, it's less clear. Calling a 20-something a 'girl' might be viewed offensively as diminutive. Calling college students 'women' can also seem awkward casual contexts (it's perfectly fine in most).