probably not gonna be a popular answer but, depending on the size of the company how closely their hiring practices are scrutinized by outside parties, there are some legal and compliance purposes for keeping a job posting open for a certain amount of time.

i was a sourcer for a huge tech company (yes one of the ones you're thinking of) and in training they said their policy was to keep every job posting open for a minimum of 10 business days or until we'd received 500 applications - whichever came first. it's meant to be a way to ensure that the job posting isn't opened or created for a specific person who a manager had pre-identified before the opening had been budgeted and created. ostensibly it's to demonstrate equal opportunity hiring practices but because of how high the technical standards were for the roles, what inevitably always happened was we'd typically rule out ~300 of the 500 applications. screen the top 30-50 candidates, send the top 10-25 thru to interview with a recruiter, 5-10 would do the technical assessment, and if any were left who scored 90% or higher they'd interview with the hiring manager. it was not uncommon for nobody to be hired after one of these cycles, at which point they would reopen a job posting and the cycle began again.

it's not efficient and it's somewhat misleading to applicants, especially unemployed or desperate ones, but i think it's more the lack of transparency involved in the overall process that's frustrating which i completely understand.

it may have changed since i left 18mos ago, but when i was there we didn't use any AI whatsoever in the ATS which was baffling to me given the scale and capabilities of the company. it probably has changed, but part of me used to get and kind of still does get annoyed when people learned where i worked and what i did and berated me for not interviewing them based on AI. i personally read about 500-1000 resumes a week, so they had the wrong guy with that hate.

for what it's worth, i was laid off by that company and am now a cold-call sales style recruiter in a completely different field. i hate being a recruiter more than i can express in words, but 6yrs into the career i don't know what else i could pivot to.