There are some understandable reasons why you might not go to FOCS. It costs money. It takes time. You have a conflict or other responsibilities. You are just not into theory.
But recently I've started to hear a new excuse.
I don't have a paper at FOCS.
I've heard students say they are embarrassed to go to FOCS without a paper. Some say they don't think they are supposed to go without a paper. Some say it's "tradition" not to go to a conference unless you have a paper to present.
I and many others went to every STOC and FOCS (and Complexity) as a grad student usually without a paper to give. It's a chance to find out the latest results, to discover the faces and personalities behind the names that you know. A place to talk to the current and future leaders of the theory community and give them a chance to know you. A chance to feel part of a large and strong theoretical computer science world beyond your own institution.
You should go to FOCS because you are a theorist and not just go only when you have the chance to toot your own horn.