Every bad hire costs the company $50,000 - 200,000. Sometimes more. They can also sink or demoralize teams.
Many of the people conducting interviews are new to the process and don't know how to extract signal. Sometimes scales don't line up.
When you have a revolving door of employees (because that's the way things are these days), have trouble scheduling interviews (busy engineers trying to get their own work done), and can't get enough skilled interviewers on a panel, then of course the process will be a suboptimal experience for candidates.
To a degree, companies would rather a good candidate was passed over than a bad candidate was accepted. Type I and II errors.
Companies also don't like telling candidates how they did because that opens them up to lawsuit liabilities.
You have to do enough interviews to get signal yet not piss off candidates. (Or your employees in the interview pool!)
It's a hard problem for companies too.