Who says the Williams sisters haven't played tournaments??
Well, Venus anyway. Since 1997 here is her # of tournaments.
2000: 10 (missed first four months with tendonitis in both wrists)
2001: 11 (shortened schedule because of 9/11 and injured wrist)
2002: 16(withdrew from fall tourneys citing fatigue)
So when Venus is healthy she plays a very good amount of tournies. Serena on the other hand, has never played more than the 13 she played last year, but withdrew from 3 fall tournies citing fatigue. So the WS don't play enough only applies to a healthy Serena.