Dropbox works with the NSA to hand over customer data. You are absolutely right that something like this would go over like a turd in a punchbowl at Dropbox HQ.
Unlike Google, Microsoft and Slack, Dropbox does have the top, 5 star, EFF rating for protecting your data from the government... I'm not sure what more they could be doing.
Their profitability depends on being able to dedupe and compress data across all customers. They would need to raise prices significantly to make client side encryption a built in feature.
Then use tarsnap already. Bonus: it is owned by our own cperciva.
tarsnap is nerdish, secure and reasonably priced for what it provides IIRC. Haven't used it though but expect someone would have yelled out here if it was bad. (In fact the only one I've seen bashing it was patio11, -because it was too cheap and too nerdish.)
https://www.dropbox.com/en/help/28