Jesus tells us the enemy will steal away the seed - the Word - from our lives if it is not received with understanding. But there's a particular kind of understanding He has in mind. These people weren't too stupid to grasp the meaning of the Word. The understanding problem was of a different kind.
In quoting the prophet Isaiah Jesus said their hearts had grown dull - Matthew 13:14-15 - "Indeed, in their case the prophecy of Isaiah is fulfilled that says: 'You will indeed hear but never understand, and you will indeed see but never perceive.  For this people's heart has grown dull, and with their ears they can barely hear, and their eyes they have closed, lest they should see with their eyes and hear with their ears and understand with their heart and turn, and I would heal them.'
This is exactly the same thought Jesus played out in His parable. He said people's lives can become hard to God's truth. Like a path that has been packed down by lots of traffic, people get busy with the thoughts of their minds and the dreams of their hearts. The seed of God's Word is just one more thing bouncing off the surface of their lives.
Remember, Jesus isn't dealing with atheists here. He's talking about how people give their attention to His Word. He's talking about the crowd who have exposure to the Word - the crowd that goes to church. He's saying hearing Gods Word is going to be a fruitless experience for many people because, while their bodies are in church, they've already become distracted and cluttered in their affections and aspirations. People can become desensitized to spiritual truth. People can actually pride themselves that they are not easily moved spiritually. They can hear God's truth in a detached manner.