have a 2000 dodge caravan why does the belt fall off when it gets wet

You may have a pulley alignment or idler pully issue. In dry conditions, there may be enough friction or grip to hold the belt on, but in wet conditions, it slips off. I would recommend having the pulleys checked for alignment. There could be a loose bracket on a pump, or the idler pulley could be worn.

When a belt wears it does not wear exactly even .. the moisture is absorbed and the tension increases/decreases on one side of the belt causing the belt to wander off the pulley. I also suspect there is misalignment in the pulleys

Also ... you did not indicate if you changed the belt ... no matter what the problem is ... change the belt

A gated idler pulley in place of tentioner pulley. Read carefully, just the pulley you need the tentioner

There are two drain lines from wiper tray. They can get clogged by leaves and debris that fall through the vent slots into the wiper tray. The drain line on the passenger side is right above the serpentine belt. If that line gets clogged that water can backup and seep out at the fitting to the tray and drip right on the serpentine belt. It happened frequently to me before I figure out the problem. Remove the lines and use a hose with a spray nozzle to blast the clog out.