Typically because people have spotted a bad player, they know already, or the table averages look good like a high players per flop number. The funny part is, that often seats will only be available, when the bad player leave the table, so going on the waiting is often a futile effort.
By the way my feeling is, that waiting lists have gotten much shorter on PokerStars after the 4 table cap. Presumably because it makes no sense for regulars to stay on waiting lists, if they are unable to add more tables anyway. This in itself has also made the cash games more recreational player friendly, because it has made it easier to get on tables without excessive waiting.
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