Guitar is defs something you can learn by internet though, but yea you're right. The technique to playing a violin is different than most instruments, just because they don't have markings for you to figure out where the notes are and you need to figure out how to tune the strings yourself as well..
Violin is more because you set yourself up to learn some incredibly dangerous habits without a teacher.
Anyone with a half decent ear could work out how to pick out basic tunes fairly quickly. It would be slow progress for most but it's certainly not an impossible task. The problem lies in that posture is not an intuitive thing unless you have strings experience. You need to be shown how to hold the instrument correctly, you need to be shown to hold the bow correctly, you need to be shown how to move the bow correctly. You only need to do it wrong a couple of times for it to be wrong forever. I've taught kids where I've had to tear apart their entire technique and start from scratch because they've had awful teachers at first, or adult learners who think they'll have just a quick play a few times to see what it's like before they start lessons.
It's true of any instrument to a certain extent, I'm a decent pianist but I still have some nasty habits I picked up while teaching myself when I was younger. On strings, those habits are a lot more likely to hold you back, you arrive at a point far more rapidly where you can't make much progress until you learn the correct way of doing things.