Because 3-reports-ever has to be counted against something, and the only alternative is your IP address — which a VPN resets and an office shares between everyone in the building. An account is the honest version.
It also means your reports are yours: they stay tied to the account rather than to a browser you might clear tomorrow.
You can read a complete example report with no account at all, so nothing about what you are signing up for has to be taken on faith.