OP I think you're really seeing some things that aren't there & starting to reach.
When the pitch is in use, there are both girls & boys teams
When the pitch isn't in use, then anyone can use it. They can dance the grass off if they want.
I for one wasn't interested in football because don't enjoy contact or team sports. My dad snapped his ankle playing soccer & it was the only time ever saw the man cry.
I preferred solitary activities. I hated PE class. I enjoyed ballet class & got out of Saturday sport because had ballet.
I remember fondly going to the park every weekend in season & watch my dad play soccer. I imagine the times the fields are booked for team sports, that this is a family friendly event.
I go to lots of parks with my dogs. Since Covid our parks are packed with groups of mixed gender kids getting around on their bikes. It feels like the 80s/90s again! Boys & girls getting out & about on wheels, hanging around parks again. Bike riding is back on the scene again, which is a genderless activity.
But Why is this ? Where were they going/what were they doing before? Now they have no other options, the park is the new place to hang again.
Am sure those with tween/teen girls will be able to provide more insight as to why their girls don't want to hang at the park.
As a teen we hung at the park to get drunk, so perhaps they'll be back into the park as they grow older - be careful what you wish for OP!