This is not always the case. A two-bedroom hotel suite on average costs more than two standard rooms. This happens because the vast majority of rooms are twin/double and cheap hotels often don't offer suites at all.
At a given location two travellers would be chosing from e.g 100 options and at least some of these would be budget/discount offerings. In the same location a family of four would have to chose from 10 options and likely none of them are budget/discount.
Now consider that you HAVE to travel during the school holidays so competition for these damn 10 options increases and the price for both hotels and tickets easily goes up 2-3x.
There are some situations where a family of four would get a better price per person but most of the time it's the other way round.