Because those totally are a 100% fool proof and totally prevent STDs.
Sorry, I will now tell everyone to live their life in the most dull and boring way, never taking risks, never having fun, and most certainly never having love because you can get STD's as an adult too!
If that's your view, have fun with that.
My view is completely different. No stuff they are not 100% full proof, but Condom+Birthcontrol+SMART love= extremely reduced risk. You're making love, a very basic thing in nature, seem totally bad. You're all sticks in the mud.