In the actual letter, the signatures are broken into two groups, one for AI and closely related experts, the other for people like Hawking, Musk, etc. Both are quite long and impressive lists, with the expert list actually being longer.

Because when a man that smart starts to think about fields outside his direct expertise it tends to be worth listening to.

That and media have never ever exaggerated anything they printed.

Even worse, taking a look at futureoflife.org (the site that actually hosts the letter), I see their "Scientific Advisory Board" includes Alan Alda and Morgan Freeman.

Now, I happen to respect both men as actors/directors. I also see they have both participated in scientific documentaries (as narrators/hosts, I suppose). I could understand if they were part of a "public relations board", which usually requires some degree of celebrity power, but a scientific advisory board? Doesn't this require one to actually be a scientist or a subject matter expert?