It is common to believe in all sorts of "urban legends" because of a human tendency to use availability and representative heuristics: what seems the easiest and quickest to remember is usually the logic we settle on.
However, there are literally thousands of pieces of misinformation; such as the piece found on www.snapes.com entitled "Spiders Inside Her" .
The article describes how people are so gullible and easily fooled into believing something, and the entire myth had openly-satirical goals.
The reason for this piece of information sticks in many people's heads is because the the common phobia (or at least discomfort) of spiders is quite common.
According to THIS articled about "Half of women and 10 per cent of men have, to some degree, a fear of spiders."
This fear causes people to remember this "fact" more, because their emotions aid in the storage of LTM's. According to the Three-Stage Model, this is a very legitimate way to turn STM into LTM in Stage 2 via the "7 +/- 2 items" to remember.
Of course, there are other factors at play, but the main ones may be that it evokes such a strong emotion in people; thus, it evokes such a strong memory. And the urban legend continues...