Threat actors who phish see themselves as businesspeople, even if that business is illegal. They’re always seeking ways to maximize their profits, and with phishing, they know they can do that by better tailoring the email lure to resonate with the intended recipient.
For example, if the fake email is meant to trick an Office 365 user into entering their account details into a fake Microsoft login site, then cybercriminals seeking profit know they’ll have a higher success rate if they only send phishing emails to registered Office 365 users.