Because of the email + phone number restriction. their is at least some control over the creation of new accounts.

Depending on the security check steemit does. The 'scammers' will first need too buy another phone number. And maybe even another device, because of the IMEI number.

That being said, even then it would be more beneficial to write on a single account. Then to write for multiple small account with tiny voting power.

More then one account = More costs + less voting power