Sam & Max is somewhat a special case because it was actually invented in the 80s by Steve Purcell independently of LucasArts. LucasArts then licensed the franchise a number of years later, much like games based on movies do so today.
As with all franchise licenses, they expire. Unfortunately Monkey Island, Grim Fandango, and essentially all other LucasArts classics were not licensed by LucasArts and instead developed as intellectual property internally.
In other words, LucasArts as a company will always own the rights to those games. They aren't going to expire in a few years and be open for Double Fine or whoever grabs them first to work on.