That's how the feudalism of Pern works. Everybody is tithing the first/best of everything they produce to the next man up the chain. The dragons and their riders must be provided for, because if they don't work at peak and Thread gets through then everybody is completely screwed. So the little Cot-holder tithes his best to the local Holder who tithes the best from his industry and the industry of the Cot-holders he oversees on to the Lord Holder who tithes the best of his industry and the industry of the Holders he oversees on to the Weyr.
When working as intended the Weyrs get the best of everything and the further down the line you go the less quality and quantity is left after the tithe is made. That and miserliness has been bred into the population for several thousand years. One of the main functions of the Holders and Lord Holders is to be the prudent man at the top who knows that just because there was a bumper crop this year doesn't mean there won't be some small calamity next year, so nobody gets new shoes until they've worn through the soles of the ones they have because you never know, and the tithe must be made.
