Football and baseball and hockey teams fill their core positions with the most skilled players available. Yacht racing teams are no different. Nobody expects a baseball coach to put a totally inexperienced player in as shortstop, or pitcher, or catcher, as long as there are experienced players available. Likewise, a racing team prefers experienced racing sailors to work the pit or the foredeck. If they can't find an experienced person, they'll take on an intelligent, eager, newbie and teach him what he needs to know. Complaining about the unfairness of it, or faulting skippers because they want the best crew they can find isn't going to help anyone get on a racing boat.
There are generally two types of racing. The more serious racing happens on weekends, and that's when skippers want their best crews. On weeknights, usually Wednesdays, boats go out for the more casual beer can races. Those are the races where skippers are most likely to take on newbies and teach them. If you didn't get an invitation to crew after putting your name on the Spinsheet crewlist, your next best chance is to find out when beer can racing takes place in your area and walk the docks where racing boats are sailing from, and let it be known that you're available.
If you're not just a newbie at racing, but also at sailing, then go to your local marina and find someone who needs crew to help sail his or her boat. Often marina employees can direct you to an older person who needs help sailing his boat. A newbie can learn a great deal about basic boat handling from such a person. If you can demonstrate basic skills when you crew on a racing boat, you'll be more likely to be invited back.