Mostly because you can complete level 25 without actually using the lift function. As I mentioned in the post above, I got all the way to 38 before I figured it out.
If level 25 was unbeatable without actually picking the box up and carrying it, such as making the entire area around the box a no-portal wall, then people would probably get the idea.
People are going to take the path of least resistance, usually meaning doing more work than is necessary because its easier than actually doing what should be done. Sometimes you just have to grab that dog by the scruff of the neck and rub his face in it to make the lesson stick.
But thats just my thinking...