The entirety of the Deadlands storyline is a cycle of irony and From Bad to Worse. In the ancient past, the Old Ones chose to enter the Spirit World and close it off from the human world because they deemed magic was not worth the price of the monsters that roamed the earth. By cutting off the flow of magic, they caused the peoples of Europe to instead turn to science, developing advanced tools and technology. When the Europeans arrived in America, the native Americans were no match for the invaders because their culture hadn't developed any further technologically in the centuries since the Old Ones had sealed off the Hunting Grounds. This led to Raven being the last of his massacred tribe, and his gathering the Last Sons to murder the Old Ones and undo their work, in the belief that restoring magic was the only way for the Americans to match the technology of the Europeans. And things just kept degenerating from there, ultimately leading to Hell On Earth.
Wait, if prairie ticks can't penetrate human flesh, how can they perform a Chest Burster routine? They'd have to get through even more flesh going out that way. And if they had the raw strength to just rend victims apart to get out in the first place, why would they go down anyone's throat?