Why are US tech workers so pro open borders?

Tech workers from what context? The majority of H-1B visas go to tech workers. There are large tech employers with teams in the USA that speak an East Asian language exclusively while in office in the USA. Teams that will not employ English only speakers.

Pichai came to the USA as a student and worked initially on an H-1B visa as did Musk. Musk once claimed he would "go to war" supporting this program though he may feel differently today.

Given that much of "big tech" is staffed and run by immigrants why would we not expect big tech to be pro open boarders at least as it relates to tech employment?

Optimistically: Empathy. Care for all humans, not just for the ones in this country. It is not wrong to look at life in America, life in the third world, and think "wouldn't it be nice if everybody could have this instead of that?" (But cynically, I suspect that most of them aren't thinking through the second-order consequences of that.)

Cynically: Greed and cynicism. Greed first. The tech workers who wanted to come here have always been able to come, so open borders are not going to increase the supply of tech workers much. But it is going to increase the supply of low-level workers, which results in lower prices in a number of areas. That makes tech workers better off, because at least some of the stuff they buy is cheaper.

The cynicism part is cynicism about America. They see it as nothing special, as having no special character worth preserving, as either neutral or negative overall. So if mass immigration happens, and it changes the character of either politics or culture, who cares?

You might as well ask why US tech workers are in favour of computer science education when it clearly does not benefit them that new people learn to code etc.

I'm not sure they always are. Certainly employers/managers are pro-US tech workers because it gives them access to lower-cost talent, and many H1B's/immigrants are supportive of it because they want more people like them. This is a common complaint of Indian H1B's and something I've experienced myself, where, especially before the H1B fee, they would only seriously consider other Indians/people of their nationality.

If you go into the midwest though there's more animosity about being replaced by lower-wage foreign talent. In my experience when US-born engineers support immigration strongly it seems to be a holdout from their college years, and not something they picked up since starting their career.

I am opposed to borders on a fundamental, ideological level. They are an expression of state violence prohibiting natural freedom of movement and trade.

I don't believe that is actually a commonly held capital "L" Libertarian belief though, and certainly would not associate myself with that party.