In addition to josho's comment, which I agree with, a higher salary would also attract more people into the market. Some people might think $100k+ is already a "high enough" salary to encourage people to join the field, but the fact is software engineering is hard. Harder than most people want to work.
I try to convince people to become software engineers all the time, and I tell them all about the salaries and benefits they can expect, and most of the time they just sigh and repeat that it's too hard.
Also, living in regions of the US where salaries for engineers exceed $100k per year also means incredibly high living expenses, so higher salaries would be necessary to entice people to move from out of the area.
As it is, I know people making HALF of what would be entry-level for software engineers, but elsewhere in California, and they have a MUCH better standard of living than entry level engineers. As in, they own a 3 bedroom house with a big yard, a couple of cars that easily fit into a 2-car garage or can be left in their spacious outdoor parking space, OR on the street where there is no-time-limit parking.
This standard of living could easily run $2M in the bay area (the mortgage payment alone would be more than my friend's salary, before taxes), but can be had for about 1/10th of that elsewhere.