You're completely right and obviously many many people would want to go and see something like the good the bad and the ugly in theatres - probably enough to fill the theatre for a showing or two - but the problem is how would they know it's on? A lot of people look up what movies are on that week, but most people are aware that the big movies are ok a the moment because they're marketed and obviously that marketing costs money. What studio or theatre chain is going to spend millions (or even thousands) on marketing a 30+ year old movie that might take in a couple of hundred k at the box office.
There's a reason cinemas don't play old movies and it's not because they think there's no one who wants to see them, sadly the logistics of it are too costly (in all sorts of ways beyond just base line finances too).