Spiritual things are a mystery to unspiritual people. The Apostle Paul explained it this way: “The human mind with its selfish and self-satisfying thoughts and purposes is hostile to God. It does not submit itself to God’s Law -- indeed it cannot. So people who choose to live solely devoted to their appetites and impulses cannot please God or be acceptable to Him." (Romans 8:7-8).
What that means is that most folks operate entirely in the flesh and not in the realm of the Holy Spirit. Because of this, they cannot understand spiritual things. So, Jesus chose to reveal some of the most basic spiritual fundamentals in parables – deep spiritual lessons taught through storytelling.