NEVER INVITED OUT WITH MY COWORKERS.?

I work in a team of about 20 people at work. The majority of them are between 18 and 25 and I fit nicely in the middle of that age range. I've been in the job over 2 years so everyone knows me well by now.

They're always going out clubbing but they literally haven't invited me once. They don't invite the older members of the team but I'm into the same things as them and I'm the same age as them.

I've been out on organised work parties before and they've offered to let me share a taxi, spent time talking and dancing with me and just generally being social so I don't feel like I'm the weirdo they want to leave out. They definitely do include me at these parties and by choice plus we have a good laugh.

So why exactly would they be excluding me when they all just go out for a few drinks or to bars? I have social anxiety (I've never showed this side of me to my coworkers and I always come across as confident at work) so this is probably getting to me more than it should do but I can't help feeling like there is some hidden reason why I'm the only one they don't invite.