The minimum salary for an H1-B software engineer in Silicon Valley is $84,000, which is on the low end of market but not egregiously low. You can check out the wage requirements here[1]
The best employees are very hard to find because they're always employed (for themselves or others) and when they do go looking for work, they rarely look through regular recruiting ads (since good workers know: these are a waste of your time). You either need to poach them, target them, acqui-hire them or encourage them to seek you out.
One of the easiest ways for the latter to occur is if they simply want a working holiday (very common for employees aged 25-35). Many people will try to get a job at large companies in an area they'd like to visit. But they'll expect you to help with their working visa.
Even if you're poaching, targeting or acqui-hiring, you need to be able to bring the employees to your place of business. If they can't get a visa, you've wasted your time and money.
Do salary deltas of even 100k make a substantial difference to the bottom line for large tech companies like FB and GOOG? Of course the difference is greater than zero but as a percentage of operating cost I imagine it's relatively small.
My guess is large tech companies are eager to import foreign talent primarily because of cultural and socioeconomic differences. Who would you rather hire to maintain your legacy code bases? A skilled immigrant that's thankful for the six figure salary and will obediently complete their tasks without much fuss? Or the native-English-speaking citizen who can find a six figure salary at any of his friends' companies and will walk as soon as the legacy codebase's tech stack starts to reveal itself as less-than-inspiring?
I'm sure they aren't paying the minimum H1-B salary at places like Google or Facebook anyway. It would be odd for some employees to be making $84k and others to be making $120k.
This is not odd at all in work places. Salary has a huge correlation with the demand when you joined and how hard you negotiate. There's a reason employers try their best to keep salaries a secret.
If you find an awesome person, who you know will be great for your company, why should one have to beg the government to let them come? Finding another such person might take months, and might actually be impossible.
Really? Maybe, because it's better for the same price? Or maybe because it's the best available solution to a problem you have, regardless of price? Maybe it's because...
[citation needed]