To ask why people sit in reserved seats?

Because loads of the time the person who reserved it doesn't turn up to sit in it. I do this quite often for a bunch of reasons, sometimes I've booked an advance ticket, but so has a friend, so we move and sit together, sometimes I've booked an open return and booked seats on a particular train but end up getting a later/earlier one. Sometimes I see an unreserved seat that's better than my reserved one so I sit there instead. And sometimes people just sit in the wrong seat, or the right seat on the wrong train, by accident..

OP, you need to work on your assertiveness. So you felt unable to challenge the woman who was sitting in your reserved seat. I get it, but your next port of call should have been to go in search of the conductor and let them deal with it. No point standing there like a lemon feeling resentful.

Because often people don’t sit in the seats they’ve reserved, they’ll get on the train and if they pass an available unreserved seat sit in that (understandably) rather than walk down the train to find the one the train company has allocated to them. Particularly since people with reserved seats are often making one-off journeys, so will have got to the train with more time to spare than someone on their daily commute and therefore are more likely to have boarded before it gets too busy.

You’d be silly to sit in a seat reserved for someone else if there are unreserved seats available, but if it’s a couple of minutes before departure and the train’s busy I’ll take my chance.

because eejits reserve seats and then do not find them... or sit somewhere else.

when I am Queen of the World reserving seats on a train and not sitting in them will be a capital offence.

Because quite often the person who reserves the seat doesn't use it. If there were no other seats of suit in a reserved seat, but would always move if asked.

Because lots of the time the reserved seat person never actually gets on the train, so it is better to take chance and sit there than stand the whole way. However they should be ready to leap up and give you your seat in that case though.