Freedom of choice

The Lightroom CC app is only a small part of the Adobe Creative Cloud ecosystem. And Creative Cloud is not just about storage. It hosts your images through Lightroom Web, synchronizes images and develop settings with other devices running Lightroom CC, etcetera. iCloud simply does not support this.

Just to set expectations, there's a whole bunch of work being done on Adobe's servers, such as AI tagging and face recognition (and the list is growing by the week), so even if they could do so, they're pretty unlikely to want to complicate it further by trying to access the files on other people's servers.

iCloud has all of that and more; anyone using Apple's software and iCloud functionality would know that. The problem isn't iCloud, its that Adobe has chosen to build their own cloud features instead of supporting second-party (Apple, Google, and Microsoft) functionality.

As for "Adobe's servers", watch your network traffic. You'll see connections to Amazon AWS. I'm guessing that Adobe went to Amazon for cloud infrastructure, rather than build their own data centers. (Not a criticism, AWS and Microsoft Azure are hugely popular. Just saying.)