As somebody in an interracial marriage who works in marketing, I feel somewhat qualified to speak on this.
The reason companies do this, as always, is purely based on what works for them economically - it’s pretty simple. They have absolutely no desire to “accurately represent their customer demographics”, that is not their objective, why would it be? They are trying to sell as much stuff as possible.
It’s not that white couples are somehow “bad”, it’s that they are completely neutral in a marketing sense. Whereas, having people from minority backgrounds in your adverts does have a psychological (somewhat subconscious) benefit when trying to attract a broader audience. It is human nature to feel more connected to entities where you appear to be represented and there are people associated who look like you or share your culture.
It’s essentially a free hit, there is no real downside to having people of different races in your adverts, but there is an economic benefit, so the better question is why wouldn’t you do it?
When my wife and I see these adverts we find it quite funny mostly, because it’s just so transparently cynical. But, as the saying goes, if it’s stupid and it works then it ain’t stupid.