They have google.com, which is the brand they use towards customers. The Alphabet brand is little more than a stock ticker.

They use abc.xyz though, which is fun at least.

They have abc.xyz

Counter-counter point: It's not really used in branding either by any of the child companies. So they don't need it.

Who ever has alphabet.com will be able to sell it for 7 figures anyway, which is the point.