Data show that having children decreases happiness.
Since when were scientists ever concerned about happiness. If they were, they wouldn't be in such an often-unrewarded (financially, emotionally, etc.) profession. Scientists are happiest when they can bring some sort of meaning and structure to what appears to be chaos. Arguably, children are the ultimate form of chaos!
They also eat a lot of your time (which could be better spent doing science)
No doubt. I'd wager that the majority of the most successful scientists (in terms of publications, innovations, peer respect, etc) are that way because they devote the vast majority of their time to science and nothing else.
and they're extremely expensive (scientists don't get paid that much).
Children can cost plenty but our memory is selective; we often ignore the constant "cons" for a few blissful, and unfortunately infrequent, "pros". In the end, you're probably right that logically, people that make science their raison d'être shouldn't have children. Fortunately for society, these people do as they often encourage the next batch of new scientists...