It's just not set up for it. Reminds of Star Trek - the original and Next Generation, along with The X-Files, which had similar issues with arcs. They weren't set up for it.

So when they try it feels shoe-horned, contrived, or implausible.

There's of course shows that have the opposite problem. Farscape, Buffy, Angel, and Firefly - worked better as arc series - or letting themselves be serials. The writers were better at emotional character arcs - and struggled with stand-a-lones.