You're right, a reasonable regulator following the U.S. anti-trust regulations as they were written in the early 1900s would also require Whatsapp, Instagram, and Facebook to all be cordoned off into separate companies at a minimum.
Facebook buying Instagram and WhatsApp is essentially them buying out their competition.
Google running a cloud service is more like renting out excess capacity – which is quite common across industries. Plus there's a lot of competition in the cloud space.
You'll be <<very>> tempted to do things to the platform to favor your own services.
The incentives are so strong I don't think we've even invented things to prevent this on a long enough time scale.
If that were actually the case, then sure.
But it's not. Alphabet's Google Cloud services have whole data centers dedicated to renting to the public and Alphabet's Google and other businesses entities are their own customer.