Why is Apple forcing mobile devices to backup to iCloud?

Not sure why you have decided that a technical issue you are experiencing means that Apple is trying to force you to back up to the cloud.

Apple probably doesn't care where you back up your files. I doubt the revenue from iCloud storage is a significant part of Apple's profits.

Instead of trying to imagine Apple's motivations, you should focus on the technical issue. It is one I have never seen and have been backing up iPhones to computers for years, and also to iCloud. I utilize both types of backups. The phone must be unlocked with the passcode and then the backup proceeds. It is concerning that you have been repeatedly rebuilding these devices in DFU mode, which is very unusual for iPhones. That might be indicative of something else that is interfering with the backup. The fact that it occurs with multiple devices might mean something in the way the Mac is configured is a common factor.

Are the devices connected directly to the Mac, or are you going through an adaptor or hub? Have you tried different cables? Do you have any security third party software, antivirus, "cleaning" type programs or utilities installed?

Do you have iMazing installed?

iMazing will prompt your iPhone for its password in order to do a backup.

Sync your iPhone, iPad, or iPod using your computer

Use the Finder to sync your iPhone, iPad, or iPod touch with your Mac - Apple Support (CA)

Excerpt from above

" Sync over Wi-Fi instead of USB

When your device is connected to your Mac via USB, you can set up your Mac so that in the future it syncs over Wi-Fi instead.

1. Select your device in the sidebar of the Finder window, then click General on the right. 
2. Scroll down and select “Show this [device] when on Wi-Fi.” 
3. Click Apply.

You can now unplug your device from your Mac. When your Mac and device are on the same Wi-Fi network, your device appears in the Finder sidebar, and it syncs automatically over Wi-Fi whenever it's plugged in to power."