Bots
Submission bots post the same text to hundreds of forms. Directory owners treat this as spam, add captchas to stop it and delete what gets through.
Autofill extensions
An extension stores your details and fills each form when you open it. You still visit every directory, pick the category and press submit. It cuts the typing and leaves the hours of clicking.
Manual services
A person reads each directory's rules, adapts the description and submits. It is slower per listing, and each listing is written for the directory it goes to. This is what StartupSeen sells.