Here's why you won't:
Chances are your pregnancy isn't something I think should be celebrated. Are you in a stable, long term, committed and good relationship? Are you financially stable? Are you emotionally and mentally stable? Are you an independent adult who takes care of herself and is completely capable of taking care of another human being? Do you have health and dental insurance? Do you have a safe and comfortable home?
I could go on and on, but here's the point- if you answered "no" to any of those questions, I don't think you should be having a baby. If you can't properly care for yourself, you're not in a position to take care of a baby or child the way that baby or child deserves. Roll your eyes, get angry or defensive, insert cruel insults, whatever you like.
In my opinion, your child deserves a life more stable and happy than you're able to provide and at the end of the day, your child's quality of life is too important to me for me to go throwing out false compliments or congratulations.
Boom. It's out there. In the world. A little scary but completely honest. Real talk Monday.
PS. When wonderful people I know announce their pregnancies the story goes a lot differently. There's a lot of celebrating, jumping up and down, crying happy tears for them and congratulating. This approach of not congratulating a pregnancy only applies to situations in which that person isn't in a position to properly care for a child.