An important note: public key cryptography allows secure communication over insecure channels, where an adversary only has the ability to listen in, but not over untrusted channels, where an adversary can actively intercept messages and edit them.

The internet is an untrusted medium, since packet switching requires packets to travel through routers that can edit them at will. (The fabled man in the middle attack) This is the whole point of the certificate issuer public-key infrastructure, where issuer public keys are included in your OS's installation files, a secure communication channel. (If you can't trust your OS, you of course can't trust any communication made with it to be secure anyway)