They can easily get more engineers by simply increasing salaries.
Let's say I'm a bright student deciding whether to study software engineering or plumbing. When I graduate, my salary will be driven by supply and demand. As a software engineer, I will be competing for jobs with other software engineers, many on H1B visas coming from countries where education is less expensive than in the US. So those software engineers can comfortably accept a lower salary. No such problem with plumbing, so it is rational for me to chose plumbing.
Now Zuckerberg and co can justifiably say "look, there is a shortage of software engineers, let's lift H1B cap".