I also wonder how much of it is also #1 data gathering, #2 advertisement, and #3 slightly counter to what you are saying - bad tools/people using said tools.
for point one, I can learn a lot on what microsoft is doing by pulling resumes from microsoft workers, even more if I interview them. you can quickly glean direction, head count, tech stack, etc.
for point two, tech companies especially have to remain relevant in the news. Imagine if you post positions for great ML shiny positions, that gets seen by tens of thousands of people for 0 cost. "Did you see that great posting at company X? wow they have an impressive Y team"
point 3, I have seen recruiters get thousands of resumes for a position they posted on the web, really want to hire someone, but instead send people through the hiring loops because of direct contacts. Its like they search for skills, the resumes on the website dont get searched well so they assume there are no matches. its baffling to me this happens given the tools you mentioned