The main reason Dropbox exists (along with WhatsApp) is that Apple is unwilling to support its "unique" ecosystem features (iCloud, iMessage) on other platforms

The main reason Dropbox exists (along with WhatsApp) is that Apple is unwilling to support its "unique" ecosystem features (iCloud, iMessage) on other platforms (they only make it a "feature" of their own platform to increase its perceived value and attract users). So if you want cross-platform, you have to use 3rd party stuff (Dropbox, WhatsApp).