The short and simple reason: Rules.
But if you're more curious: Payments regulation laws require more extensive identity verification for specific payments. This is usually for payments above a certain minimum, but it can also happen when it's unclear who the money is going to or coming from. These are called "Know Your Customer (KYC)" laws.
In these cases, we need more ID information before releasing funds. This could be your full 9-digit Social Security Number if you only entered your last 4 or your photo ID. In rare cases, we might ask for other documentation to verify your residence.
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