The issue with Dropbox is that they also have access to your encryption keys, which means they can easily decrypt and access your files, at their discretion.
According to Drew Houston (Dropbox CEO), they need access to your files to offer features like search, to be able to better understand how you're using the service, ability to integrate with third-parties, and for law enforcement. Some of these "trade-offs" are mentioned by Mr. Houston himself in this interview when responding to criticism from Edward Snowden a few years ago: https://techcrunch.com/2014/11/04/dropboxs-drew-houston-resp...
More to the point, giving Dropbox (and their affiliates and trusted third-parties) permission to access to your files is a key provision of the Dropbox terms of service:
Our Services also provide you with features like photo thumbnails, document previews, commenting, easy sorting, editing, sharing and searching. These and other features may require our systems to access, store and scan Your Stuff. You give us permission to do those things, and this permission extends to our affiliates and trusted third parties we work with.
As Mr. Houston said in the article referenced above, if you want better encryption there are alternatives.
Disclaimer: I work at Sync.com