Then any new person who enters the field sees the rates and just accepts it (both employer and employee).
“Sure go walk, I can see what everyone else is paying”
A bunch of competitive companies can (don't say collude) mirror each other's salary ranges and maybe they're OK with not using higher salaries to hire away talent which may or not actually be worth more money anyway. Of course, this happens to some degree anyway as companies research this sort of information.
Suppose that you are among the top performing workers in the labor market. If you have two job offers, you could get those two companies to match each other's offers.
Each of the two company's best response is to match the other's offer to the point at which their offer is exactly equal to the value that you would bring to them. At that point, your net value to them is exactly zero, and beyond that, your net value is negative. So the highest salary that you can get is the value that you would produce for the company where you would be the second most productive.
If either company tries to stick to the publicly posted salary ranges even though you would bring them more value than that, then they are forgoing profits and not following their equilibrium best response.
... are you one hiring people ?
Companies already do research into what other companies pay. As another poster noted, if there is a non-competitive equilibrium (mostly among a certain class of competitors), there can be a tacit agreement that we just do whatever Google does. No formal agreements needed. Of course, companies already have better information than public websites do so they can mostly already do this without narrow salary bands being published.