And with strikes, weather related closures, maintenance closures, etc I end up having to rebook my ticket at the last minute.

I'd say half of the advanced purchased journeys end up costing me more than just booking the train the day of.

Yes! Now we are getting on to the true cost of advanced tickets.

Fines for making mistakes, rebooking fees, rebooking and having to pay a higher fare, lost tickets because sometimes it's just easier to book a new ticket, losing out when unwell etc...

I bet they make a tidy sum from unused advance tickets

(Great Britain specific):

Although worth noting when it's out of your control the railway does have to offer you options. You don't have 100% of the liability.

And if your train is cancelled you're entitled to take the one before/after for example or get a refund

Though they do make this difficult these days