Stacey is right. They don’t jump into bathtubs in any sort of fatal attraction way. One way of thinking about and dividing groups of spiders is whether or not they can run on smooth surfaces, even upside down. Some can, some can’t. It’s a way of separating families Trochanteriidae and Gnaphosidea, for example. read more
Why you so often find them there is that once a spider has fallen into a tub, it is extremely difficult for them to crawl out. Porcelain's combination of hardness and smoothness make it very hard for a spider to crawl out. Spiders require minute imperfections in a surface to be able to climb on walls and/or ceilings. read more
House spiders are thirsty creatures living in a very water-poor environment, and any that venture near a sink or tub with drops of water in it will try to reach the water, often by climbing down a wall. read more