MPEG-LA doesn't claim to have all possible H.264 patents in their patent pool. Nokia has an FRAND obligation for H.264, so it's obligated to license on fair, reasonable and non-discriminatory terms, but it has chosen not to be part of the pool.

MPEG-LA did not claim to have all possible VP8 patents either.

Note that MPEG-LA is basically a group that sets up convenient one-stop licensing, there's no such thing as being a "member" per se. You can be part of any of their patent pools, or not, if you believe you have essential IP.