Why have my so called "parents" been lying to me all this time?

I was supposedly "adopted" back when I was 5 years old by my aunt and uncle. We lived together like a normal family and from what little I can remember about my child hood I led a fairly happy life knowing the whole time I was adopted. Well, now I'm 14, and from the information I've pieced together from conversations I've had with my "mom" and "dad" I've found out that I'm not really adopted. My birth mom never signed any paper putting me up for adoption, she only signed a paper stating that it was ok for my aunt and uncle to take care of me without adopting me. So in other words, in stead of being adopted I'm actually in foster care. Now I'm ok with being foster care cause I know that if I wasn't here I would probably either be dead, or in an even worse situation. All I want to know is why have my parents, or should I say my foster parents, lied to me all these years about me being adopted? Protecting me doesn't seem like a very logical reason to lie to me. I mean I was happy as a child even though I knew I was adopted, why wouldn't I be happy if I had known from the start that I wasn't even adopted? They could of very easily explained the whole situation to me in terms that a child could understand. "Your mommy and daddy need help and can't take of you so we will be watching over you." "Mommy and daddy are going away for a while and can't take you with them so you will be living with us until then." Something like that. Did they not think I wasn't smart enough to figure it all out or what?