I work in a school, communication can be somewhat last minute even for the staff so I can understand the frustration. Teachers also have families and so also experience short notice of events in the schools their own children attend, so we do get it.
Sometimes it's because as @Fourandthedog said, its to do with priorities. Emergencies happen (like children being taken into care, or an emergency safeguarding meeting, or an incident has happened at school) and the Headteacher can find themselves getting swallowed up with that stuff. Or it may be that if any outside agencies are involved, that things can change and have to be rescheduled due to staffing shortage/illness.
If it's events like a fundraising, or an assembly, these just have to fit in as and when, around everything else that goes on. Before I started working in a school I had no idea of what else happens and the pressures within a school to support it's pupils, not just in academic education, but also in social/emotional support too.
I would however politely feed back that you would love to attend and show support for the event, but that you would need more than a weeks notice to do so, and could that be taken into consideration for future events?
(As school staff, I don't get to attend my kids events during school hours, so I do sympathise).