Why commerce interception had to avoid blockading neutral ports

Why not blockade neutral ports instead of intercepting merchant ships at sea?

Blockading a neutral port risked war with its owner, so ships trying to return home had to be intercepted after sailing.

Blockading a neutral port risked drawing another power into the conflict. Enemy merchant ships attempting the homeward voyage could instead be intercepted after sailing. Remaining in a neutral port is treated here as leading to internment, although the conditions governing that outcome are not explored.