You're thinking about it wrong.
Imagine you don't really like chocolate and someone is trying to get you to eat family-size bars of Dairy Milk 3x a week, saying "But everyone loves chocolate??? Are you sure 1 Freddo a month is enough?". You'd say - well, different strokes for different folks, not everyone likes chocolate.
But when it comes to socialising everyone assumes the default is 'the more the better'. And it's really not, but it means everyone assumes you're weird when you don't like parties or big social events.
I don't enjoy them, I don't find it rejuvenating seeing lots of people, I don't come away thinking "Ah how nice!". I come away knackered and irritable and glad it's over. Why would I or your friend willingly go and do things I/she would find stressful/tiring/annoying?
(I'm maybe less extreme than your friend - I will go to wedding and parties if I have to/it'd be very rude not to, but I would 100% rather not and will turn down anything that won't cause an issue).
I get it if you enjoy socialising, but I find it so strange that it's assumed that everyone does enjoy it: there's plenty of us who don't!
I spent the weekend alone - DH was away. One friend who lives round the corner rang me on Friday, Saturday AND Sunday to invite me various places with her family, so concerned about me - she genuinely didn't understand that not only did I not feel lonely having a whole weekend without plans, I actively didn't want any and would prefer to not see anyone. It wasn't even that she thought it was odd, she couldn't comprehend that being an actual point of view. And I find that MUCH weirder than people being introverted, to be honest!