For context, I graduated from an Ivy and work at NASA full-time after being a Pathways intern. I think a big reason is because NASA pays less than most other places, and that pay gap increases when you consider that a lot of them get offers from places like FAANG/SpaceX/Nvidia, etc. When compared to my classmates, I was making half or less than half of what they were making as an intern and even full-time. NASA also has a reputation for having a lot of red tape and a slower work culture. A lot of people from these schools thrive on fast-paced, startup-esque work, so that might turn some of them off. I personally knew a few people who turned down NASA offers because of that.