The whole internship thing for STEM careers is to onboard you early while you are still cheap. Even if you don't work for the company that you interned with, the progress is still there so on day 1 you aren't totally lost. No one wants to give a salary to someone or even spend the money on the hiring process that hasn't worked a day in their life because they have zero clue how well they get along in a work-environment vs. an academic one. From an HR and hiring manager viewpoint, even if you have a lot of experience with a certain tool or field but have no work experience, you are going to look a lot less desirable than someone that maybe has a little bit of experience in that field or with that tool but mostly has unrelated internships. There are lots of smart people out there that can't get along with anyone so they suffer in their career.