The problem with implied copyright is that the majority of work is not intended for profit. Implied copyright gives at least 70 year protection to every trivial thing - every written email, doodle, picture and since people don't typically put a permissive license on everything they create that they don't intend to profit from we're left with a ton of work with hard to find authors that can't legally be used by anyone. It also means that services that deal with what people write (which is most online services) have to grant themselves lengthy terms of service conditions in order to operate.

Basically most work is intended for public domain, but not licensed permissively so it's lost.