Signal requires phone numbers because of taking care of users' privacy. Signal's choice to require a phone number is due to storing a contact list on a local machine to protect users' metadata. And PGP is never as secure as Signal or Other Encrypted Messenger (WhatsApp). Many Cryptographers blamed about weak security of PGP (https://latacora.micro.blog/2019/07/16/the-pgp-problem.html, https://latacora.micro.blog/2020/02/19/stop-using-encrypted...., https://blog.cryptographyengineering.com/2014/08/13/whats-ma..., https://blog.filippo.io/giving-up-on-long-term-pgp/) and even Edward Snowden doesn't use it now (https://twitter.com/Snowden/status/1175437588129308672).