Believe it or not, it can be easier. Often you're seen as a certain type or level in your current job. They can't imagine you doing a different one. Plus, if they promote you, they just have to fill a lower position. A new company doesn't have any of these biases.

A friend in night school said he fully expected to have to move to a new company to get promoted. His current boss didn't see him as someone with a masters-level intellect.

Of course it's not always this way. Etc. Etc.