It is simply the best/simplest solution and trying to prevent users from passing and integer here would be like opening a can of worms. Because:

* we cannot make mapstructure validate our duration string ( with an UnmarshalJSON func etc.)
* we cannot make mapstructure spit a string instead of a duration and packer will decode-encode-decode config.
* the hcl2 generated code asks for a string, so this will be enforced by default.