Why is a landing approach speed kept above stall speed instead of being set as slow as the airplane can possibly fly?

Answer: Keeps control margin

Keeps control margin ✓ — Right. Landing speed is slow, but not as slow as physics permits; it preserves margin for gusts, small corrections, and controllability near the runway. Transport-category rules define a reference landing speed above the stall reference, so the target is a buffer, not the cliff edge. That margin is why 'slower touchdown' is not always safer.

Makes touchdown softer — Not quite. A soft touchdown depends on flare timing, sink rate, and control, not simply carrying extra speed above stall. Too much speed can even make an airplane float down the runway. The reason for the margin is more basic: before the wheels touch, the airplane must still have enough airflow over the wings to respond.

Keeps descent shallow — No. Approach speed can interact with glide path, but it is not kept above stall just to make the descent shallower. A stable descent can be flown at many angles depending on power, drag, and configuration. The key reason is that near the runway the pilot needs control margin for gusts and corrections.