Why does clickbait use numbers?
Digital Marketing often produces 'content' for users to consume. 
That content is ment to catch and keep the attention of the user, in an attempt to make the user trust and follow the sender (usually a website or blog). 
When the user shows engagement like clicking the right buttons, the user will somehow be tempted with some kind of offer (a newsletter, a free ebook, etc). If the user takes that bait, there will later be an offer to actually buy something.

The whole idea here is to catch and keep the users' attention.
One of the most common ways to achieve this, is by making lists of something the user might want to read\see. 
In order to have users click(open) the content to begin with, the writer needs to make the content seem valuable in only 1 short line.
By typing the number of entries on that list, the writer is trying to make a point: There's stuff to see her, come check it out!