For the same reason a plane holding for IFR release cannot take off until
the IFR arrival he's waiting for has either cancelled IFR or has landed.
Not true at a controlled field.

Assuming IMC, what does a controller do to ensure separation in the air
for the departing aircraft from an arriving IFR aircraft who has not
cancelled IFR or landed?

Or is your comment limited to VMC?

Jose
r.a.s trimmed
--
Get high on gasoline: fly an airplane.
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