It's just plain practicality. If you have a large group that exists in secrecy, it's unavoidable that they'll need to interact with the rest of the world in one way or another. I don't remember any kind of binding spell to keep them from talking, but if you have a witch or wizard entering into the magical community from the muggle world, you're going to have to deal with the family.
It's possible you could use spells to create a false perception, for instance, casting a spell on the Grangers to make them think Hermione is just going to a school for the gifted, but that would not be fair to them or to Hermione, since Hermione would be limited in what she can tell them about her new life and they would not have a fair or clear picture of the choices made with their daughter's life. With the Dursleys, there's almost no way to avoid it, considering everything that's happened within their extended family.
If they are, in some way, bound from talking about the magical world with others, then there's no real advantage of hiding the truth from them if they will need to be closely involved with someone in that world.
It's also possible it's part of the agreement made by muggles and the magic world that involved the witches and wizards withdrawing and remaining hidden. If part of that is a magic agreement to not deceive families of witches and wizards, then there is no choice but to be honest.