Perhaps they should have; psychologists and others I have talked to over the years generally urge adoptive parents to take this path.
But rightly or wrongly, your adoptive parents chose to conceal this from you, and I'm sure that at the time they felt they were doing the right thing. Perhaps they were afraid you might feel they didn't love you as much as they would a biological child of their own; perhaps they were afraid you might feel rejected or inferior because your birth parents didn't keep you.
But whatever their reasons, I urge you not to be angry or feel that they somehow failed you. Listen: You aren't an inferior or second-class child because you were adopted–far from it! Your adoptive parents chose to make you part of their family! They wanted you and loved you, and you were very special to them.
If you have never done so, pause right now to thank God for your adoptive parents. Remember: Bitterness and thankfulness cannot live together in the same heart. Where might you be today if your parents hadn't adopted you? Where might you be if God hadn't led them to you? The Bible says, "Give thanks in all circumstances, for this is God's will for you in Christ Jesus" (1 Thessalonians 5:18). And if you have never asked Christ to come into your life, do so today–and when you do, God will adopt you and make you part of His family forever.