1.  A player can enter up to 6 tournaments in one week.  They monitor the lists to see which will be the best choice taking into account whether they will make main draw / Qs / alts and how easy / expensive it is to get there.  Clearly that means withdrawals in up to 5 other tournaments for each one accepted.

2.  Entry deadline is 3 weeks before the tournament starts with withdrawal deadline 1 week later, but in practice most players enter well before that (so that they don't forget) based on which tournaments they can get to / are likely to get in to at the time of entry.  Their ranking may well move between entering and the entry cutoff / withdrawal date which means they may raise / lower their sights nearer to the tournament and withdraw from some and re-enter others.

3.  At the lower level there is very little planning of schedules in the way that higher level players are able to do.  There is a lot of last minute decisions based on where you can afford to attend / get into / stand some likelihood of picking up points.

4.  Because they often enter well in advance, by the time the withdrawal deadline comes around maybe they are a little bit injured or have the chance of a good training week or whatever and then withdraw from all of the tournaments they have entered.

Edit: Added point 4