Having gone through several rounds of interviews with over a dozen companies a few years ago, I can tell you how frustrating it was NOT to get feedback especially when a) they offer the option in their correspondence and b) you feel really good about the process and want to know what went wrong so you can correct it. Sometimes it only needs to be as simple as we found another candidate that had more experience or better fit for the position. In the end it came down to two companies where I had a good overall experience and they gave feedback throughout the interview process. I suppose it can be a lot for FAANG companies as they are dealing with hundreds of thousands of applications a year but it should be something that smaller companies strive to do in their interview process.