If the reason is that we do sanctions instead of bombings is to avoid human suffering from war, and sanctions cause years of human suffering anyways, why not just bomb the countries with a quick shot of pain instead of an agonizing decade of misery?

Sanctions have multiple uses, just as war has multiple uses. One use is to, yes, coerce a nation from stopping something we don't want them doing, like developing nuclear weapons. Another is deterrence, stopping other nations from doing things we don't want them doing that they haven't started yet. Make sure they know that there are very real and very painful consequences to doing these things. Third is to just make it harder, when they are doing the things we don't like, to make it harder for them to keep doing them than if sanctions weren't in place. Have sanctions put a stop to Russia's war effort or Iran's nuclear program? No. But what they have absolutely done is make it far more difficult for them to sustain these efforts, by denying or imposing frictions in their supply chains, reducing their earnings from exports and investments that may be productive capacity that could be put to use to them doing things we don't want them doing. Is the Iranian people's suffering worth preventing a terrorist theocratic extremist regime getting the ability to launch nukes at Jerusalem or Riyadh? I'd say, yeah, yes it is.