It's also incredibly hard to stop taking antidepressants. A family member was on SSRIs for less than a year, and decided they weren't helping. In the process of trying to wean herself off them she went through long bouts of nausea and vomiting. She said it was like being seasick for 3 months.
Antidepressants can be a lifesaver, but they can also have some terrible side-effects. They should only be used as a last resort.
It depends on the person and I guess on the particular SSRI as well. In my case, I could (and did) go from normal dose to zero immediately, and I wouldn't notice any difference until few weeks later when my average mood and productivity started to visibly deteriorate.
Anyway, SSRIs are relatively safe - that's why they're deployed as entry-level antidepressants. There are many "heavier" options available. Getting on an SSRI is far from last resort.
Withdrawal and dependence varies massively from person to person as you say. I had been taking Tramadol (an opioid) at the maximum allowable daily dose for close to a year, and was told to stop by a new consultant. He told me it will be "hellish" and "worse than quitting heroin"; I stopped taking it immediately (without tapering), and the only effect was the return of the original pain.
Antidepressants can be a lifesaver, but they can also have some terrible side-effects. They should only be used as a last resort.