Q: Why can't we build public networks that don't require VPN to be safe?
Is there any area of your life that is “safe” from the get go? Conversations you have in the privacy of your home can easily be overheard. Phone calls are recorded. Even trusted friends and family will occasionally rat you out from a secret.
Privacy/security were not design parameters for the creation of what was to become TCP/IP and the internet. As there is no foundation upon which to build strong encryption, we now require “bolt-on” solutions such as VPN, PGP, S/MIME, SSL, etc.
With that said, there is an opportunity available to you - get the education and career position in which you can build that foundation!