Dropbox mentions Microsoft, Google and Apple as competition, because for these companies, file synchronization is just another feature (not a product). Steve Jobs said it himself: https://techcrunch.com/2012/01/23/dld-2012-drew-houston-yes-...

And sure, the big three don't seem to care too much about privacy (although Apple has recently been making some strides with regards to encryption).

The fact that there are many "not worth mentioning" competitors building Dropbox alternatives (or add-ons) with E2E encryption may signal that there actually is a demand.

It's only a matter of time before one of these companies gets it "right".