We're talking about carrying grain to feed a few millions people for a while. The gateways the Asha'man can make are most often too small for more than small wagons, and unless Rand asked the 47 Aes Sedai to help (which he wouldn't do, he doesn't want to be in their debt), they couldn't link to make really huge gateways (that would have sped things up some, but not enough to match what the SF could do. There isn't likely enough big transports in all of Tear to carry grain for millions in less than several trips). It would have taken them weeks at least to bring all that grain to Arad Doman like that, and Rand had pressing needs of the Asha'man for other tasks, like setting up on the Illian border, and the others coming with him to pacify Arad Doman. That's why he asked the Sea Folk to do it, and it took them all their big ships to meet his demands.