Once you have a paying consumer, you can then up-sell them into different brands, rather than spending $XX per new customer.
Today, they all spend money, individually, acquiring (likely) the same customers on the same platforms. Bidding against each other ends up increasing CAC's for everyone.
1. Imagine you're among a big crowd
2. You stand on your tiptoes for a better view
3. Then, everyone begins to stand on their tiptoes
4. Now you're worse off than when you started
That's user acquisition.