The fact that they're used for general purpose use-cases and the fact that they're hosting applications for a wide set of other businesses which are threatened by possible anti-competitive actions of platform holders.

Xbox is a device that hosts video games and media services. A phone is hosting all those and hundreds of other use-cases, from banking and financial services, to hardware access via IoT interfaces, car integration and myriad of other applications.

Phones are many many people’s primary computing device these days (they have no laptop or desktop). And such a general computing device is increasingly required for various parts of life (school, interacting Witt various government agencies, business, all sorts of leisure uses (social media, etc) that are core to a person’s identity)