There are many reasons why your posts could be getting more likes or engagement than your campaigns. You may have family, friends and acquaintances who see posts on your page and like them. This could be because they enjoyed the content, found it interesting, or because they are showing support.
When you are running paid campaigns, you are marketing to a completely unknown audience. It’s usually difficult to get likes when your audience is not familiar with you. Therefore, you should continue running campaigns to familiarise your brand with your audience.