Most people are focusing on modern reasons. The historical reason is that when the internet was being developed, public key encryption hadn't been invented yet

Most people are focusing on modern reasons. The historical reason is that when the internet was being developed, public key encryption hadn't been invented yet. The 1960s and early 1970s was when the internet was created and it was only until the mid 1970s that public key encryption came about. The problems of secure key exchange hadn't been solved at the time. Even if those problems had been solved, clock speeds were measured in megahertz [edit: sometimes kilohertz]; every clock cycle mattered. Fast forward to today and this legacy has remained. Many protocols are being retrofitted to support encryption, but it's similar to why railroad tracks are the width they are: Roman chariots used the same width. Additionally, don't discount developer laziness. It's much easier to debug a plaintext stream than an encrypted one.