Because Apple differentiates their products by their storage sizes, they also sell iCloud subscription. There is zero (in fact negative) incentive to respect your storage space.
Been a while since I needed to use it there but it always amazed me that the Windows implementation of iCloud was more flexible in terms of location and ability to decide what files got synced.
Ho ho, except for where it puts the photos. Those go into a subfolder of the system photos folder, and there's no configuration (yet you can configure the "shared photos" location)
And then, should you try to set up OneDrive (despite Microsoft's shenanigans, it does simplify taking care of non-tech-savvy relatives), it will refuse to sync the photos folder because 'it contains another cloud storage' and you'll genuinely wonder how or why anyone uses computers anymore