Why can't you define a table alias on an update?

This is the query I posted a performance question on, but I have a
SQL standard question about it too:

   update missing_ids
   set mtransts = a.mtransts,
   mtranmemtp = a.mtranmemtp
   from memtran as a
   where a.mtranmemid = missing_ids.mtranmemid
   and a.mtranseq = missing_ids.mtranseq

Why can't you define an alias on the primary table in an update query?

That would make the where clause in the above a bit easier to write
and read, since both the primary table and the table it is being updated
from have the same column names.
--
Mike Nolan