Not everyone comes to this conclusion, one line of reasoning is: because there shouldn’t be a meaningless universe in the first place, it must actually have meaning.
The belief that the universe must have a purpose is a trap. It leads you down a particular line of reasoning that ends with you believing in a creator god (or gods), or that the universe itself is divine. You would likely also reason that you have a purpose in the universe that dictates your actions (adherence to religious dogma or doctrine). All of this is baseless and rationalized without objective evidence. Your only certain "purpose" in life is that which you, yourself make.