Likely because people believe they'd happily give up salary for more flexible work, but really wouldn't, and so you'd not get any employees if you didn't pay high salaries and offered more flexible hours and some culture benefits.

I think Tikhon's point about figuring out how to show that you're offering the benefit is key.

If my salary is $200k, it's clear what that means, and I can be pretty confident that there won't be weird social games to get me to actually accept less this pay period. (There's an argument that income tax is an example of this, but that doesn't vary in unpredictable ways between positions.)