How many Comp Sci or related discipline students do the US universities graduate every year? How much is the demand? How many of those graduates are from other countries and need VISA to work?

How do you know what skills Microsoft needs and why do you assume they pay slave wages?

I don't think you're fully informed. Sometimes it's difficult to get qualified applicants at all, at any salary level. A lot of developers are more interested in the specifics of the job, product, company etc. than the salary (once it meets a certain minimum floor); and for other jobs, the specific knowledge and experience needed is very scarce. If there are 20 people qualified for a job and 21 open positions, no salary level is going to fill all the positions.

I had a H-1B visa once upon a time, but I never moved to the US. The company wanted me to, but I preferred London to Santa Cruz. Didn't stop me working for the company remotely, paid out of their UK sales office. Now I don't know what proportion of H-1Bs are like that - a global search to fill a local position - but it's definitely non-zero.

(The reason I'm pushing back on this is because I don't want HN to be a happy home for the same sad sack of devs Slashdot hosted when I last frequented there some years ago; "dey took our jerbs" was a constant refrain any time H-1B came up.)

I could not agree with you more. It's literally part of the business model for consulting firms here.