They don't. But when the local newspaper food reviewer gives you a glowing review and there's lines out the door waiting for a table when you have full covers for the night, and they happen to drive by /their/ building and see this, it doesn't take a rocket scientist to figure out you're doing well. The unfortunate reality of restaurant economics means you could have booming business and still making little or no excess profits though, depending on how adept you are at controlling other business costs, but since the landlord can't see your books they use these other indicators to decide to fuck with you instead.
There is neither a legal nor inherent natural requirement that a landlord choose a reasonable or accurate metric to decide to raise your rent. In fact, in most parts of the country (world?) raising your rent is an entirely arbitrary decision in their full discretion. You seem to be under the impression that the just world fallacy is a truth, when in fact it's not only untrue, most landlords are scum who will happily do as much financial harm as possible to you to the very edge of the limit for what it takes for you to go out of business. The landlord doesn't want you to go out of business or move, which is the only incentive tempering their greed at all.