2 reasons:

1) even though zaps are all public data and information, i didn't want to give people easy tools to pop in any npub and look at someone's earnings. it's not privacy, but a slight deterent.

2) features are coming that will require an nsec to sign events for DMs, Zaps, and badges to contact and thank your supporters.

Friends don’t let friends paste nsecs..

I wish there was an easier way tho, like zap to login, or DM to login, or paste a npub and get a DM with a login link..