Actually, some places do not require preordering, but they are the minority I believe.
From the Girl Scout Website:
I've moved, and now I have to pre-order my cookies. Why is selling and buying cookies different from one community to the next?Each Girl Scout council determines its precise method of helping local Girl Scouts sell cookies to local customers. Cookies can generally be purchased through the following types of sales technique, or hybrid thereof:
Pre-order: Girl Scout councils provide participating girls with an order card to collect orders from their potential customers. Girls turn in their order cards, the council orders the cookies, and then girls go back to the customer to deliver them a few weeks later.
Direct sale: Girl Scout councils avoid the order-card process and provide girls with cookies to sell directly to customers.
Booth sales: Girl Scout councils allow girls to sell cookies at booths set up inside and outside various retail establishments. Booth sales usually occur during the last month or so of cookie season.
Online and mobile sales: In 2014, two Girl Scout councils—in Houston, Texas, and Minneapolis, Minnesota—will offer girls the ability to sell cookies online and from mobile handheld devices. This new sales channel is only available to selected girls and troops within the jurisdiction of the participating councils. Girl Scouts of the USA hopes to expand online and mobile offerings to more Girl Scouts and consumers in future cookie seasons.
Girl Scouts participating in the Girl Scout Cookie Program may only sell cookies according to their Girl Scout council's policies and procedures, and within the council's published sales timeframe.
https://www.girlscouts.org/program/gs_cookies/faq.asp