I agree with Arfur, who agrees with Sloppy.
It comes down to responsibility for people's lives and a significant amount of materiel.
That responsibility should not be brushed under the carpet, especially by someone who has no more responsibility (and accountability) than checking and responding to their emails at work.
How many times do we whinge about no one losing their jobs or fronting a court to account for massive losses or fines?
99.9% of the time I'm happy to run a relaxed flight deck on a first-name basis. But if and when the need to exercise my legal responsibilities arises, the salutation of "Captain" should inform people of the position I hold and it's rights, not least of which lies in the Tokyo Convention.