There are times when remakes can be good.
The problem with remakes, I think, is that different people expect different things, thus it's impossible to satisfy everyone. Some people may want the remake to be just like the original, but others may criticize the movie as a result, perhaps regarding a 100% exact reproduction (except with different actors and better special effects) as a sign of lazy directors.
Some people may want the remake to resemble the original, but also be its own story, with some things changed around so that the remake is still able to throw a few surprises here or there. But some people seem to hate this type of remake, stating that there should be more of an attempt to make the remake like the original.
All in all, I think the real reason remakes don't do as good as originals isn't because the remakes are always bad; it's because people expect too much from remakes. However, if people were to stop comparing remakes and originals, and start to expect a remake as a new movie with a new story rather than be 100% faithful to the original, remakes may do better, in my opinion.
If I had to choose, I'd want a remake to still resemble the original, but at the same time, also be different, with some aspects changed, so that it actually feels like I'm watching a new movie rather than a 100% replica of an old movie, just with graphics enhanced and actors changed.