Merriam-Webster literally has a "vulgar" tag next to each definition. But conventional usage precedes dictionary usage, and it's shared convention that "shit" is a swear word, regardless of how a dictionary classifies things.

And it is absolutely not used the same way as "shoot". It carries a great deal of additional meaning. The entire purpose of swear words is to not be appropriate for polite conversation. Otherwise we wouldn't have them.

> Why?

Because it's unpleasant to come across gratuitously offensive things like smelly bodily functions. If I'm reading an article about economics and business policy, I don't want to be interrupted by something unpleasant that is totally unconnected to the subject at hand. I don't want a close-up photo of a pimple being popped either, as a gratuitous visual analogy for how resources are extracted from an economy. Does that satisfy you?