Why do people keep saying,"you can't change people" when you can?

I think the phrase refers to people who don’t want to change. You can’t change someone who doesn’t want to make the change themselves.

People only change if they really want to, finally get sick of whatever they are doing that is not lending itself to good results.

If someone is ready then you may have some influence but it is not because you changed them, it’s because they are ready to change.

People can change.
You can not change them.
They have to change themselves.

What @TheOnlyNeffie said. People choose to change and sometimes they read a book or enter rehab to help make that change. However, the person has to want to pick up that book, enter that rehab, etc. All the books, documentaries, fitness plans, etc. in the world can’t help someone change unless they first decide they want to. Saying that you can’t change people doesn’t mean that people can’t change, just that they have to do it themselves.

I’m a cognitive behavioral therapist working in another capacity. Shaping behavior is what I do…that and art. The quote idea you are talking about is superficial. People make up all kinds of superficial platitudes to explain the world around them. They aren’t always accurate. All species can be trained. They can be rehabbed to some degree also. There are sometimes few limitations to this and there are sometimes great limitations to this process as well as resistance to it.
But the comment also suggests that the focus for change needs to be on yourself not the other. The reason for this is that you need the cooperation of the other person for change to occur. Of course you can shape behavior by dancing differently yourself. You step differently, you interact diferently and it forces the other person to also move differently. But it can also result in acting out “change back” behaviors.
Let me give you an example:
My coworker is sometimes skeptical when I tell her what my next step will be in regards to a student’s behavior. Sometimes what I do appears antithetical to the goal. For instance most people have a hard time addressing an issue in a state of conflict and then allowing people to act out and doing nothing themselves…or appearing to do nothing. Nothing and refusing even negative attention can be a very effective means of breaking a pattern of negative interactions. And waiting for the other to come to you can be a way of setting the stage for real change.
But you really have no control over the other….they have to come back to you. Some don’t.

It means you can’t change other people. The only one you are in charge of is yourself. You can suggest or even insist they change, but if they don’t want to, there is absolutely nothing you can do about it.