Because to actually have those types of conversations you have to have legitimate experience. To be a bit flippant, here's a relevant xkcd[0]. To be less so, "in groups" are pretty good at detecting others in their groups. I mean can you not talk to another <insert anything where you have domain expertise, including hobbies> and not figure out who's also a domain expert? It's because people "in-group" understand nuance of the subject matter.
That's one interpretation. But that interpretation is still dependent upon intra-group recognition. The joke relies on the intra-group recognition __being__ the act of bullshitting.
[0] https://xkcd.com/451/