Perfect fifths are a bitch to play on the violin indeed. You need to cover two strings with one finger, which needs to be placed at just the right angle. And it

Perfect fifths are a bitch to play on the violin indeed. You need to cover two strings with one finger, which needs to be placed at just the right angle. And it's very obvious when you are slightly out of tune, unlike single notes. But pianos don't exactly use equal temperament, either. There's a curve applied to equal temperament, which makes everything sounds more in tune. At the end of the day, musicians follow their ears. Whatever sounds the best is the most important.