The first answer is because they're utterly idiotic! They're thinking they're doing the people getting married a favor by hiding a few things just so that they can start a new life, that it's no big deal to start a relationship based on hidden truths and lies. But hiding crucial information has long term consequences no matter how much to try to convince yourself that nothing will happen, truth always has a way of catching up to you, and when it does no one suffers more than the two people bound in that new relationship!
And the second answer is that I don't think Aarti will find out about the truth until Prashant himself comes back, or maybe somewhere down the road just when her relationship with Yash starts getting better! It's not going to happen before the wedding, because there is no way Aarti will agree to go through with the marriage after knowing this, so I think this is going to be a long term drama reveal and will play out in a way that will leave Aarti in a very uncomfortable situation!