It's because "tu dois" means specifically "you" (personally). Using "il faut" is impersonal, meaning simply that it will be necessary for someone to do it. Using "on doit" uses what we might call the generic "we", the generic "you", the generic "they". It does not assign the task to you personally, but somebody is going to have to do it.
Thanks for replying!
Wouldn't it be better to render it ("on") as "someone", then?
DL really often gives us prompts with "we" in English, yet "we" almost always refers to, well, us in English, while DL expects us to use the English pronoun "we" as if it were the French "on", itself the source of the English passive/generic "one" - and "on" is practically never translated as "one" despite it being the closest match.
(And never as someone/somebody.)
I am also not quite sure I got what I was most curious about - about 95%+ of DL exercises dealing with obligation/must/having-to/etc. utilize "il faut", so does this mean the French just practically never 'accept' personal responsibility or just never speak in a manner accepting/imposing personal responsibility?
Ie. do DL's give a good picture of how French is spoken, or is DL just really obsessed about teaching us the "il faut que" formula (to the exclusion of actually used phrasing), even though it is not used anywhere near as often as one might think based on DL exercises?