Can my boss tell me I am being anitsocial when I can't or unable to attend a work social event

Of course. It doesn't fall under the set of civil rights (in most countries, don't know where you live) that shouldn't be violated in the workplace, regarding things like race.

All that matters is what the consequences might be if you don't go. Are there any?

Stick and stones may break my bones, but words will never harm me.

While I don't completely agree with that sentiment, it does mean that your boss can voice his opinion about your actions. He can certainly say he thinks you are being anti-social, by not attending. What he can't do is take some action against you.

Yes, most real career advancement comes often from social events. While not required, it is often deemed important

Huge grey area here... unlikely you will get fired for this specifically... BUT it will cast your career in a negative light... while social events might not technically be Mandatory, in many jobs this is part of team building... and it will make you appear to not be a team player. True or not... sometimes appearance is what matters most. And if you are an At-Will employee... they can pick any number or reasons to get rid of you eventually if they see you as not fitting in with the group.

Most peoples excuse for "not being able" to attend are actually "not wanting to attend". Enough that's what is assumed when its not a one time event.