> Wouldn't everyone be copying all the best ideas from everyone else? Wouldn't that be a good thing? Both in general and for innovation?

No because this incentivizes copying over innovation. There is little reason to invest in R&D when anyone can copy your idea. Consider the COVID vaccines. Each cost billions in R&D. If they could be copied then a company that specializes in production could invest all its money into that and spend nothing on R&D. Eventually no one will be willing to invest in innovation until someone comes up with the idea of protecting IP again.