And many musicians have lost copyright cases, because — per my point — their shit was similar enough by complete accident, because
1. We Live In A Society™ and therefore are constantly exposed to all sorts of things we don't consciously remember 5 minutes later but which stick around subconsciously, lurking for your next “Eureka!” moment; and
2. Musical genres (especially the mainstream ones) tend to coalesce on rather limited sets of chord progressions and rhythms and such, so it becomes more and more difficult to write songs that fit within that genre without accidentally “plagiarizing” some other existing song in that genre (or a related one).