Sure, there are plenty of adult options (bars? Most pubs after 8pm?).
Many pubs rely on the revenue from families - coming for lunch or early dinners. Without that crowd, I doubt our local would survive.
Ultimately, this is about behaviour. So long as children are well behaved, I think the vast majority don’t mind their being in pubs. The issue is the small number of people who think their child can run riot wherever they are - and banning children is easier for these venues than individually addressing badly behaved punters.
I often take my children to the pub. I like to have a social life still, they are well behaved. I would not tolerate running around, shouting etc.
Like it or not, it’s not the 1970s anymore and rather than being adult spaces, most pubs are community social spaces for anyone who wants to use them. And - they need the money from anyone who uses them!