Valve turned a huge profit on Portal 2. Yet they refuse to make Portal 3. Why? Because compared to Steam, Portal 2 generates a tiny ROI by comparison.
Option A: invest $1 in Steam and get $10 back
Option B: invest $1 in Portal 3 and get $2 back
which do you do?
Same goes for movie industry. A movie promises returns for decades as it is milked and re-released in various formats. If the movie can only make money for 20-ish years, ROI goes down significantly. Read: "The Disney Vault" [1]
May I offer an alternative reason why Valve did not make Portal 3? It's because when you rehash the same game, people get sick of it. Sometimes a product needs to stand on it's own. Why harm the Portal franchise by being to quick to release, release, release?
Seems like hollywood is already on board with this. What do they need Ghostbusters to have a 100 year copyright on when they could just release a new Ghostbusters every 30-40 years like they're pretty much already doing.
Like I made mention before, just because they don't want to release it now doesn't mean they won't want to do another one in 20-30 years after technologies and ideas have had enough time to mature to put a new twist on things.