Because I can send a message to a public key, encrypted such that only the owner of the private key that pairs with that public key, can decrypt. Since the user actually owns the keys, they can't get rugged by Google or Microsoft because they find themselves on the wrong side of big brother. Public on-chain wallets as identity are also much more spam-resistant. Verifying a cryptographic signature is completely decentralized and does not require even internet access (could be done fully offline) whereas email is tied to a server by definition. Numerous possibilities open up with public-private keys.
I have dozens of wallet addresses. In fact, while typing this message, I just created 3 more, just for fun. It's really as simple as running an ECDSA function in the browser if you want to create separate identities for separate platforms. The beauty of owning your online identities is that you are in control. It's really that simple. But if you want to compose that identity, there is really no better way.
We've already seen deplatforming, censorship, financial exclusion for protesting in a democratic nation, shadow banning, and worse from the big tech cos. That's not the internet or the values I was raised on, and I'm working to push back on it, vis-a-vis tools like SIWE (https://docs.login.xyz/general-information/siwe-overview/eip...). Take ownership.
An email address? Why do we need to complicate this with blockchain tomfoolery?