To wonder why single people don't get invited to things.

[Basta]

Just that, really. Dinner parties, holidays... Even in my immediate family, my parents (a couple) are currently on holiday with my sister and brother-in-law (obv a couple), while I'm on holiday on my own somewhere else.

Is it just a question of dynamics, or something more sinister?

Both.
YANBU.
I've been single for years and I've seen this happen a lot.

No idea but I completely know the feeling.

Yup completely agree...

I'm single and I get invited to lots of things

Try being divorced

Next time crash the party. If you want to, Until you crash it with your +1. Don't let them go away with it. Life is short. They might think you want to be on your own while hunting... It's usually in movies that everybody knows your +1 before you do... Have a nice vacation anyway.

Do you invite people to dinner parties and holidays?

I didn't get invited as a single, don't get invited as a couple either! Some of my single friends go out loads.

I got invited out more as a single, I think as part of a couple people assume you’re covered for the whole ‘things to do’.

I do get you. I have found post divorce it is a lot harder to do certain activities eg parties or dinner parties. You get overlooked :(

Well I think you need to ask your parents why you weren't invited. It could be any number of reasons.
I'm married nd have never been to a dinner party either OP

Depends on your circle of friends. I hardly ever go out as a couple and most of my friends come out without their partners or are single.
I can't think of any events that have been specifically for couples although sometimes my DH tags along.

I'm single and don't get invited to much. Spoke to my mother on the phone this morning, she mentioned going to Greece with my younger brothers, I said Oh that would be nice but not sure I can afford to. Awkward silence. They didn't mean me too. Just them and partners.

Do you invite people to dinner parties and holidays?

Fair point. I guess I don't expect a couple would want a third wheel on their holiday. I've made a few suggestions in the past.

They didn't mean me too. Just them and partners.

What did you say, Vampire???

I've found that being the only single adult in my immediate family means that while I do get invited to things, I'm the one most likely to be expected to sleep on a sofa. I understand the reasoning. Doesn't mean I like it.

Likely they don’t want you to feel a spare part. If it’s all couple then just you by yourself, they all go off to their rooms to get ready/go to bed leaving you on your own just feels mean on holiday but is the reality. Dinner parties again discussing your plans as couples would just feel rude.

I don’t host or share parties/holidays anyway so I leave everyone out regardless of relationship status.

Sometimes in friendships groups it might be because they are insecure and believe you will “steal” their partner but that’s their issues not yours.