Why doesn't Pakistan occupy Afghanistan and make it part of its territory because it would benefit Afghans by availing sea access and benefit Pakistanis by availing direct trade with Turkey?

 They tried, but they failed in curbing Afghanistan’s freedom.

There was a guy named Ahmed Shah Massoud. He and his Northern Alliance crushed all the hopes of Pakistan to maintain its control over Afghanistan.

Ahmed Shah during Afghan civil war 1993.

After the Soviet’s withdrawal from Afghanistan, a power vaccum was created. Pakistan seeing the opportunity to maintain its dominance over Afghan politics filled the power gap with Taliban. Gulbuddin Hekmatyar and his Talibani millitia was backed by Pakistan in Peshawar accords. Pakistan backed Taliban in each and every aspects whether funds, arms or intelligence.

They were so close in getting dominance all over the Afghanistan but Ahmed Shah Massoud never gave up. He and his Northern Alliance never gave up like true Afghans. Taliban was so fed up that they were forced to sit on a table and make terms with him. He rejected all the terms made by TalIban and stated

“We will never be a pawn in someone else's game. We will always be Afghanistan.”

This dialogue reached the doorsteps of Pakistan declaring that Afghanistan is and will never be under their control.