A scholarly take on the guest worker visas from Civil Rights advocate Norm Matloff ... [ http://heather.cs.ucdavis.edu/h1b10min.html ] ... major attraction for employers, especially in Silicon Valley, is the "handcuffed" status of H-1Bs ... There is no tech labor shortage. No study, other than those sponsored by the industry, has ever shown a shortage.
"Tech labor shortage" is not a clear-cut black-and-white issue. The market is not fully elastic to easily use terms such as 'shortage', etc.
Say, you need to build a team of 5 machine learning scientist with specific experience in image recognition, there may be 100 people in the country specializing in this field, and perhaps 10 of them are the job market, and 7 are international citizens. Then you have a serious case of shortage, and face the possibility of losing the project/business/market to a company in Israel, India, or China that has the requisite staff or to a company in Canada or Australia, where immigration laws are more sensible.
At the same time, there may be no shortage for generally skilled engineers fresh out of college.