Why is a table with a drawer that partially pulls out muktzeh,
But a pocket sewn in a man’s trousers does not make the trousers muktzeh?
It is actually the other way round.
If drawers pull out completely this means that the drawer is a separate entity from the table. If the drawer has muktzeh inside, the drawer becomes a basis for the muktzeh item and the table a basis for the drawer (unless there’s something more important that isn’t muktzeh on top of it).
However, if a drawer doesn’t pull out completely, it is part of the table, just as a pocket is part of a pair of trousers. In this case, the pocket or the drawer alone become muktzeh, and the trousers or table do not.
See Shulchan Aruch and Mishnah Berurah 310.