The first one is something I've been saying for years. I worked in computational biology for a while and it took me a while to realise, but eventually I noticed I wasn't very good at it. The people who were good at it lived and breathed biology. They would come to the lab on weekends and even the middle of the night to water plants and generally tend to their experiments. Above all, they cared . I was just a computer…
All this advice runs into time constraints and luck, which means you can get unlucky trying 2, 3, 4, 5, etc. things and you're not good at any of them while you still have bills to pay.
It's also an accessibility problem as some careers are gated by degrees (physician) or capital (farming). Doing the sample work (shadowing a physician, hired farm worker) to see if you like it has a risk of not giving a real picture of the actual work. If that happens, you have to rely on tenacity to stick it out.
Some people need to be in the deeper parts of the job before their brain kicks on and starts enjoying it (just being a hired laborer at a farm vs. owning and running the farm) because they don't have any 'ownership' when it's just a job.
All this to say that the quick advice like the OP is technically right, but it has about the same nuance and considerations of reality as clubbing baby seals.