Open less food, serve smaller portions but more often and do not refrigerate leftovers. Left in the can at room temperature with the lid loosely covering the can the food will still be in great shape 2-3 hours later, good enough for them to want to eat it again.
The only other thing you can do is put the food out and forget about it.
It will not go bad, and hours later it will still taste and smell a million times better to them than food you've tampered with in an effort to preserve it.
They don't mind if the food gets crusty, we do. We are the ones who think crusty food is something terrible. It isn't.
They care about smell, taste and texture. Whatever you do with the food, you change all of those and that's why your cats will not eat such food.
When you carefully examine refrigerated food, the first thing you notice is that it doesn't smell the same any more. That alone is enough for many cats to reject it.
Adding water means a major change and watered up food (even watered up fresh food) is a huge big turnoff for many cats, even some that are otherwise not finicky.
Etc, etc, etc.
Basically, it's not leftovers that cats don't like. They will happily eat up food you leave out at room temperature.
When you refrigerate, warm up, water up food, the changes in smell, taste and texture become very noticeable and because it's so different, such food is no longer palatable to them. And so they refuse to eat it.