What’s a backup going to do? It’s not as if a database backup would enable business to continue the next day. For a data model the size of Xero, the logic of the application is inherently tied to the data.
In the event of catastrophic data loss at Xero, you could at least take your debits and credits, transactions, invoices and other basic accounting data with you to another vendor. Sure, Xero adds plenty of application logic, but at the end of the day, it's just double-entry accounting.