Because they might have missed it by a tiny margin and were borderline. The feeling of totally bombing an interview and being within an inch of passing are enterily different. I wouldn't be upset by the former, but the latter would grind on me for a bit.
To the grandparent poster: if you actually have the goal of passing that faang interview, just keep trying. Taking multiple tries to succeed doesn't make you a subpar engineer. I worked for one of those FAANGs for a while, but there was another one i wanted to work at, and I kept coming short every single time during onsites for that one specifically. Like, it would always be good enough to go to the hiring committee, but it would inevitably end up with "you were so borderline, only if that one interviewer out of 5 gave you a slightly higher score, you would have been hired." Turns out, 4th time was the charm, and I got hired. And now, I am not noticing being "subpar" to any other engineers i work with who got in on the first try, my perf review was quite good, and I like the job I do, so trying all that time ended up being worth for me. So if you have that as your goal, don't get discouraged by failing, just give it another try later.