I fundamentally don't think governments should do a careful cost-benefit analysis of everything in society and then ban it if it falls on the wrong side. Just on basic principles of personal freedom. That's why the "addiction" framing is so important, because it implies that citizens don't have agency, and so justifies the authoritarian intervention.
PS if we apply your analysis to video games they surely would have been banned too.
Edit: by the way I remember back in the day we searched for "addicting flash games" and it was seen as a positive ;p
With social media, the cost benefit analysis doesn't deliver marginal results, just less stark/concentrated results. Drink driving is self evidently bad even though 99 times out of 100(?) it does no harm, because one time out of a hundred its consequences are catastrophic. Social media on the other hand is harming essentially 100% of the population in initially milder ways - even if you don't use it you're forced to live in a dumbed down society where wealth and power is becoming concentrated in the hands of those who pedal digital dopamine and in a democracy being undermined by disinformation. Of course 'initially milder harm' is step one in frog boiling.
[1] https://contact.dvla.gov.uk/driver/capture-transaction-type