You can read The real story of how the Internet became so vulnerable but the real story is simple: It wasn’t designed with security in mind. Every bit of security has been bolted on after the fact, and it shows.
Fixing it would require literally replacing everything right down to the physical layer. That kind of upheaval simply isn’t going to happen. An alternative network is an option, but whether or not people could or would even want to switch to it is an open question.