They don't have to call around the company blindly hoping to get the right person. With the possible exception of the reference manual, the consultant relations program's services are free. Since vendors can't offer them to every curious high-school student who has their phone number, they may impose qualifications on whom they support. These qualifications are seldom arduous, and never get in the way of a genuine consultant with a legitimate need. If vendors didn't provide this central service, getting information would be a chancy proposition.
Many consultants call local sales offices.
Fine - if you have a specific client whom you can identify, have convinced a salesperson that your client is a qualified prospect, don't mind dealing with a different salesperson for each client, like having your requests drop off the priority list when a hot prospect calls, and don't want to be updated on an ongoing basis. A central team, whose only aim is helping you, is usually more accommodating. Nobody sets up a program such as this out of the goodness of their heart. Charity is nice, but doesn't translate into operating budgets. Vendors expect something for their efforts.