Good question. When you do a post on Facebook Page, on average less than <10% of the fans receive your message due to multiple reasons:
Fans not online when the post is done and by the time they come online their newsfeed is replaced with other messages from the pages they've liked or from their friends.
Facebook's newsfeed algorithm decides to share your post on the newsfeed of selective fans, those who've shown interest in your page by various kinds of engagement such as like, comment & visit
Based on the above factors, the value of a Facebook Page has significantly come down and you are forced to pay to Facebook to take your message to your fans :(.
Does this answer your question? Let me know if there's any follow up question.