Essentially because ICANN says so. ICANN requires the registrars to maintain accurate contact info, and also to publish the contact info on port 43. In addition they allow registrars to offer proxy registration service, which does has additional requirements for the registrar (see the whois primer linked in this thread).
HOWEVER, due to the GDPR, ICANN's "Temporary Solution" to the GDPR changes a lot of these requirements. One of the changes is that the requirement to make the contact info publicly available via whois has been removed. Registrars now are only required to show the state+country of the registrant, as well as some anonymized way to contact the domain owner (e.g.: via an anonymous email, or, via a web form).
For transfers of domain names between registrars, this also means the gaining registrar no longer is required to email you asking for authorization to transfer the domain (because the gaining registrar does not have access to the email addresses any longer). ICANN is working on a replacement for whois called RDDS which has tiered, authenticated access, as well as an accredidation process for people that need access to the real contact info such as law enforcement. However, they haven't worked out all of the details on this yet.
ICANN has also said regsitrars are not required to distinguish between an EU customer or a non EU customer when applying this rule.
So essentially, ICANN's temporary solution to the GDPR makes a lot of the reasons for "Whois Guard" or proxy registrations less useful.
See: https://www.icann.org/resources/pages/gtld-registration-data...