It seems to me they already tend to offer quite high salaries.
Further increasing them could increase the number of available programmers, although there are going to be both short-term and long-term availability limits. And obviously, companies can't afford to pay arbitrary high salaries.

More specifically, I suppose that much of this labor demand comes from startups, which often operate on the brink of financial viability.
Startups have high failure rates, but a few of them generate a very high return on investment, which is what makes the whole startup industry viable: VCs are as risk averse as anybody else, but by diversifying their investments in many startups they reduce the variance of their return and thus obtain a positive expected utility. However, if failure rate goes up (for instance due to increased labor costs) without the other parameters changing, it would kill the whole industry, and I would expect this to occur in a very non-linear fashion, essentially as a threshold effect.