I haven't actually noticed this, but I can think of two possibilities:

1) The developers wanted the snakes to make a sound - one they'd expect people to associate with snakes. Most snakes make very little noise (and many try to make no noise at all) and if they do it's a quiet hiss, which players might not recognise as anything to do with the snake. So they went with the one sound they probably think of as obviously associated with a snake.

2) The developers are American. One interesting thing I learned when teaching environmental education in the USA is that many Americans believe that all venomous snakes are rattlesnakes (they're not, rattlers are only found in the Americas, and there are many other venomous and non-venomous snakes there too). More rarely I'd meet someone who thought all snakes are rattlesnakes. I actually had a few adults interrupt my classes to insist that cottonmouths and copperheads, or even the coral snake I was holding at the time who clearly did not have a rattle on her tail, were rattlesnakes.

They're aspiring rattlesnakes

Partner, I don't know what ya're smoking but I ain't very seen a red striped raddle snake.