Simply because the law was written that way. But also the whole idea of identity verification becomes pretty useless, if there is no chain of trust. You could run a modified client that lets you assume any identity you choose, exactly the opposite of what eIDAS is trying to achieve.

This is necessary because the wallets contain an identity proofing functionality called PID(Person Identification Data). Showing these credentials basically approves you are you. There are high requirements for identity proofing that even pre-date wallets and that makes sense, because the potentially blast radius of identity theft is huge. Historically, these have been secured in smartcards, like eID cards or passports and are not shifting to the smartphone. Verifying the security posture of your device and app is therefore crucial.