Open source benefits big tech too. Eg see all the huge corpos who donate to the Linux Foundation, because Linux is necessary to their work. Google gets good PR from summer of code, and students will be recommending summer of code to each other. It gives Google a good amount of cultural capital among engineers. It's not like Google has no cash to spare for a couple projects to make them look good and secure their role as one of the most dominant tech companies across the field.