It is at least in part because of how recruiting works, especially in large tech companies. Companies have many candidates for each role, and can't easily review all of them. Between bureaucracy, how busy the hiring manager is, etc. many candidates get lost in the process.

This is why 1) it really helps to have a referral and 2) tell the recruiter when you have competing roles. Neither will change much about getting or not a role, but they will really help the prioritization and avoid you getting lost.

Paradoxically, that effect is bigger nowadays when the market is not as hot as it used to be, because recruiting is more stretched. Even some FAANG are very understaffed on the recruiting side