It comes down to greed, there’s always someone who wants something for nothing. There are only twelve notes in music and some chord progressions have been used in so many songs that justifying a plagiarism claim must be very difficult. When I learned to play guitar, I was told that if I wanted to write a successful song (!) use I V vi IV (first, fifth, minor sixth, fourth), it works in any key and more hits have been written based on that progression than any other. Some of these plagiarism claims are ridiculous, as I said, comes down to the envy of greedy people.