Talk me out of just calling you when something breaks.

Break-fix has an incentive problem built into it. Your provider earns only when something is wrong, which means nobody is patching, monitoring, verifying backups, or watching for a compromised mailbox in between. Those are precisely the tasks that prevent expensive events, and in an hourly relationship nobody is paying anyone to do them.

The second reason is financial shape rather than total. Hourly costs arrive in lumps and they arrive at bad times, because equipment fails when it is working hardest. A server dying in your busiest week is a large invoice on your worst possible week. A monthly fee is a smaller, predictable number you can plan around.

The honest counterargument: if your business can lose a day without much harm, and you already have tested backups and multi factor authentication in place, hourly is defensible. If that describes you, we will say so.

Note the condition in that sentence. Backup is not included in an hourly arrangement, so unless you have bought it separately or set it up yourself, that counterargument does not apply to you.