It's a legitimate worry and I feel the same way. But email, in spite of all the spam and problems, is still widely used. Standards like DKIM, SPF, DMARC have mitigated those issues. I feel like nostr's path will be similar to SMTP as a protocol.
Discussion
DKIM, SPF and DMARC all serve to prevent account spoofing, which isn’t a problem for Nostr and it’s priv/pub keys. The real problem is the opaque structure of mail providers, each with different spam filter mechanisms and preferential treatment towards “safe” origins (gmail, apple, microsoft, aws ses, etc…) encouraging a closed system