An inbox is hard to ignore because the reward schedule is uneven.
You can open it 10 times and find nothing useful, then hit one urgent reply or welcome invitation on the next check.
That surprise teaches your attention to come back again, even though most checks have little value.
The habit is not really about email. It is about giving your brain repeated chances to win a small unpredictable prize.
Fixed checking windows remove those chances. The inbox becomes a tool you visit, not a slot machine you keep feeding.