you could ask the same thing about hamsters! or stamps or fish or cats, dogs, horses, sheep, rats, GIANT SQUID!!
but these are my reasons.
1: theyre beautiful! they come in lots of colours patterns, shapes, sizes, with legs, without legs,
2: theyre unusual, exotic, interesting. i wanted a break from the norm eg, hamsters,cats.
3: easy to care for (most of the time) they dont need to go out for walks, they dont get muddy pawprints all over the house! they dont make much noise/no noise at all!
4: theres a huge variety out there each with theyre own specific needs, so i guess its a challenge.