Most likely because there is something wrong with them. I refuse to accept that its normal human behavior to be obsessed with a pregnant women's stomach. Or to be so bold that you ask a complete stranger if you can touch them.
The older women, however, I will accept it as part of their normal behavior. But only to a certain extent. It's undoubtedly nostalgic for them.
If he was awake he might kick and they'd be able to feel it. In a few weeks you might even be able to visibly see where he kicks if I were to allow them to see my bare stomach. Which I won't.