Why? This doesn't line up to the reality of how headcount is allocated or how budgets are set. Every position has a range the hiring manager can place the new hire in given their experience and fit.
It prevents companies from wasting peoples' time going through a whole interview loop, claiming a job is 200k-400k and then offering 200k-250k regardless. If the 200k isn't attractive to job seekers, they have to raise the floor.
If the budgeted range is 200-400k (it's very likely not) then it's an accurate post. It also shows the applicant that they can get 300k or more if they have a good argument for it beyond "I want it."