Because it's a convenient lower tech solution where you don't need to be a merchant with access to the debit card system to get monthly (or one-off) money, and it's easy to take or give back money. As other comments say, it's also trivial to claim back a fraudulent charge, it's exposed directly in every online bank system UI.

In Switzerland the concept of a "Einzugsermächtigung" exists too, but it's a step or two more than just knowing your IBAN.

Basically you do this with your bank and not the other way around with the merchant.

Is it common to check all your banking stuff every month to make sure there is nothing fraudulent? I don't see how this is practical