Because many people are not great at entering their email addresses correctly and many sites don't require any sort of address verification/confirmation.

If you have a common word or common name email address at a big email provider then you almost certainly are getting: password reset emails, billing invoices/order confirmations, tax info, childcare/education notices, medical appointment confirmations, local government notices, business conversations, wedding invitations, etc.

All legitimate and not spam but intended for a different recipient.