Why doesnt my girlfriends parents allow me to be alone with my girlfriend atleast for 5 minutes?

Im 16 years old an a junior in high school and my girlfriend is a sophmore and 15 years old (turning 16). Now my question is why in the world will her parents not let me be alone with her (just me and her) for atleast five minutes!!! i understand im a teenager and all of that but to be watched over like freakin rats in an owl den? her parents seem to like me and they take me places with her but theyll never EVER let me be alone with her to talk about things. What should i do ??? simply wait until we turn 18 then say screw you were gonna be alone???