I think the S&P is overvalued today. The market thinks otherwise. No matter how much I would like to purchase reasonable stocks at P/Es of 15, by and large, I can't. If someone else wants to buy them at P/Es of 21, they are going to attract every single seller.
https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...
a) These companies are much bigger than they were, which increases the impact a programmer can have. This means the amount of money a programmer can bring in for one of these companies is much higher than previously, and so they're willing to pay more.
b) Previously, programmers were capturing a much smaller share of the value they were creating for their employers. Either through explicit collusion to keep employee salaries down (https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...) or because the threat of startup founding (which is both easier and more normal relative to 10y ago) has pushed salaries up.