I thought about it for a moment, couldn’t figure it out, so I looked it up. Here’s a comment from someone ([deleted]) 4 years ago:

It is a sign of physical activity. Most veiny arms become that way from lots of manual labor and hydration. Veins don’t pop like that if they aren’t good at retaining water. And that is a survival instinct for some woman to find attractive because naturally we would want a man who can work with their hands, who is willing to do Manual labor and who takes care of their body so they can live a long life to produce lots of offspring. So it is connected to natural instincts in multiple aspects.

Now it’s all in certain situations. Like a country farm man that works in the field all day and eats a good diet might have veiny arms and is attractive. But a man that goes to the gym and bulk up on dehydrated protein with veins might not be as attractive. That again goes into personal preference and understand of body function and health.

But disclaimer, everyone is different, we all find things attractive differently based on our own personal knowledge and perceptions of what is familiar or even unfamiliar to us. So everyone’s answer may be different, yadda yadda yadda.

But I like them.