If you have to install it, though, developers can't count on it being available to all or even most users.
Those are two different markets
The point is to sync application data between native apps running on different (and even different kinds of) devices. PIM-style data (calendar, contacts, notes, bookmarks, and so forth) probably comes first for most people. Apple has also added useful stuff like Wi-Fi passwords and E-mail account configurations.
And then developers can create their own entries in the iCloud data store for their own apps. This is hugely useful.
I'm not aware of any similar facility that comes with Android, but I'd be happy to hear about it if there is.
Regardless, though, if it's not built into the OS, developers can't rely on it being present on a majority of users' phones.
Did they even really try?
As far as iCloud "alternatives" go... Android doesn't offer ANY legitimate syncing infrastructure to compete with iCloud, open or not.