I don't think it is different.

Companies that are large/sophisticated enough to have formal policies of employee conduct generally ban a wide range of behaviors that prioritizes the employee's personal gain over the company's.

I suspect you're imagining a single relationship in your head, and imagining that it could go well. Yes, indeed it could. But you have to think at scale. If you have ten of these sorts of relationships in your company, the odds of one of them going kerflooey in an expensive or damaging way is effectively 100%. Heck, the odds of five of them going kerflooey in an expensive or dangerous way has to be at least 90%.

You can't write policy based on imagining single incidents. At scale, power dynamics will be abused, full stop.

It's a problem for the company even without misconduct. If a coworker claims a promotion went to a romantic partner instead of them because of romance, the company has to spend a bunch of time looking into that, even if there was no misconduct.

It's the same thing with conflicts of interest. The clear possibility for misconduct is a problem that at least must be declared, and at best is avoided.