They don't wait because they don't know how many LOR are comming.
After they verify your GPA it takes (according to them) 4-6 weeks to mail out your application (aside, some people have it mailed out quicker then that).
Only the letters that are already there get mailed out. The rest of your letter will be mailed as they come in, AADSAS said it takes about 2 weeks from the received date for them to be mailed.
The best bet is to call them and cofirm all this.