Dropbox never claimed end-to-end client side encryption. That said, neither do any of the popular cloud drive options - for example Google drive or One drive.

End to end client side encryption is a tough problem from usability. Most people don’t like being told they can’t recover their “cloud backup” if they lose their laptop or phone or forget their password.

I think even iCloud only does end-to-end for specific sensitive data.

There are special providers like SpiderOak who do this, but it is non trivial to use safely.