I have them too. I get them on my thighs, so I have these lovely blue veins when I wear a bathing suit
I've had sclerotherapy a few times to help reduce them. Sclero is a cosmetic procedure you can have done at your dermotologist's office. It's basically a saline solution that they inject into each vein to break it up and dissolve it back into your bloodstream. It does work, but some stubborn veins require more than one treatment. And new veins will always develop, so it's not a permanent solution. But it's the only thing I've found to have any success.
If you have red veins on your face (I have those too, near the eye area) your derm may recommend laser treatment. I haven't tried that one (yet lol).
Not sure why I get them. Probably genetic, although I don't know anyone in my family who has them. So it's probably some great grandmother or something.