Simply because, my dear chap, any pilot who has just "lost it" in front of his passengers is hardly in the right frame of mind to be flying anything, never mind operating a commercial flight.
The safest thing is to opt not to fly. If that is undesireable then a better option is to terminate any such conversation before it takes you into that mental state where you shouldn't fly.
Yes, this captain didn't just say one or two curse words to a passenger, he was loudly cursing on his cell phone before the passenger was involved.
From a news story:
The pilot of the Las Vegas-to-Detroit flight was apparently in a heated cell phone conversation in the cockpit, then went into a lavatory, locked the door and continued the conversation, Federal Aviation Administration spokesman Ian Gregor said Saturday.
"Passengers who were boarding the aircraft could hear his end of it," Gregor said.
Doesn't sound like he was focused on the flight.
Then again, I wasn't there.