Why sail-assisted cargo ships remain difficult

Are sailboats becoming a reasonable form of cargo carrier again?

Not as a primary system; wind assistance may save fuel, but ship layout, sail scale, route safety, and unreliable weather impose severe limits.

Sails are being considered as a way to improve the efficiency of powered cargo ships. Container ships already use their decks for cargo, leaving little space for conventional sails. A kite large enough to help propel a ship weighing tens or hundreds of thousands of tons would itself be enormous and could create problems for aircraft. The Ever Given blockage in the Suez Canal adds a concern about disruption on constrained shipping routes. Wind is also intermittent, which limits the case for relying on it as the main source of propulsion.