the issue here is that many/if not all of the club nights do not use specific tickets (i.e. just for that night) and instead have generic tickets for the venue, which can be used on any night at any time. the problem therefore is when everyone who possesses a ticket decides to go the venue at the same time.
further, as has been said, some clubs do become "full" although sometimes its just a case of waiting till some people leave until more can be crammed in.