Why don't my parents trust me?

My parents don’t trust me at all. I don’t see why though because I’m not the type of person that stays out till stupid times, i’m sensible, I know how far I can go without crossing the line and yet they really don’t trust me at all. Im 18 in a month and my mum basically told me she doesn’t trust me to not take drugs when I go off to uni in Manchester in Septemeber. And today when I asked her if I could sleep at my mates house after her house party they said no as they don’t trust it even though they know if it got out of hand I would call them and leave! I don’t see why they don’t trust me.