I'm not very familiar with the internal and editorial details of comic book companies, but is there a reason why the storylines don't allow the heroes to mature?

I remember when Batman married Catwoman, but I already expected them to return to the status quo after sometime. Recently, they undid the relationship between Spider-Man and MJ again.

Why don't the publishers allow, for example, Batman to have a daughter with Catwoman? Or for MJ to become pregnant and have their child be part of the canon?

Considering the recent contributions and developments in the world of characters, such as Damian Wayne and Jon Kent, who add to the mythology and create room for new stories, why doesn't this happen more often?