You get your shirt when you physically arrive and check-in. But there, you will be able to choose your size. There is not an online option to "reserve" a size.
Well, I wouldn't think they would print a unique shirt size just for you. You'd have to choose from the available options. Now, if running out of a particular size is a concern, I suppose you'd have to just be one of the first in line.
But why not? I can't express my world appearance if I can't wear the shirt. Also, there is only a limited amount of people going to worlds so why not ask everyone there sizes. This way there is enough sizes for everyone participating in the tournament.
Simple: the logistics may not merit the effort. T-shirts are just one small aspect of the entire weekend. By the time they got all the responses from everyone coming, there may not be enough time remaining to have them printed and shipped and quality assured and transported to the venue.