1) Materiality: the supply of H1B visas relative to the overall skilled workforce is too small to have a material impact. It's 0.06% of the workforce (admittedly it's a higher percentage of software developers, but still less than 1%). By contrast, illegal immigrants comprise 5% of the total workforce and probably a MUCH higher percentage of day laborers to the point where it does materially impact wage levels
2) skill level differences: Most anyone can be a day laborer, so if you raise the wage level, you can actually increase the supply in the short term and bring supply and demand to equilibrium through both increased price and increased supply. If you constrain supply of software developers and allow the salary levels to rise as a result (putting aside the fact that I don't believe H1B visas have a material impact on salaries), you can't attract more software engineers in the short term. So the only way to bring the market into equilibrium is to just inflate salaries in an artificially constrained market. Over the long run, maybe more people decide to become software engineers, but in the short run, an accountant can't suddenly become a software engineer because the salary level is so attractive.

I take it you're a software developer?