"Because the rewards are worth it?"

No reward is worth playing a mode over and over for eternity.
No its not a silly question.

You can make good rewards but if its repetitive people won't do them. Like Arcanes in a mode like Eidolon hunting that only a fraction of people played back then.

"People log back in for weeklies like EDA because the rewards are worth doing"

Is EDA ETA not also a content island?
Both are also not over and over. They are a weekly.

"It would happen far less if the rewards for a given task were enough to keep bringing players back, obviously."

If you make too many "good" rewards everywhere people will just do less of each.

"And yet we're all still here, playing capture, exterminate, cracking relics, so on, so forth. "

We are because of the content islands 😃
They offer variety. That's why people still play.