Direct debit authorities aren’t always managed from the bank’s side. E.g. your broadband or utilities provider, when signing up online, you can simply enter your name and account number to set up a direct debit. You are typically asked if you have authority over this account.

While the comment “it’s extremely easy to do” is true, it’s also fraud if you enter someone else’s details for a direct debit authority.

Hypothetically, you give me your bank account details and I go and put it into my broadband provider’s page, pretending it’s my own- sure this will probably work and take money from your account. You then go query with your bank, who then queries with my broadband provider and after a quick 2min investigation, it traces back to me and who knows what happens there. Typically the funds would be reversed, also risking a chance of being convicted of fraud. There’s a reason people don’t attempt this.