First, notice that the owner of the field did not create the problem. Interestingly, neither is the problem attributed to angels or to the wheat. An "enemy" purposefully placed the seed. Thus, from the perspective of this parable, the existence of evil in our lives was originally intended, but intended by the "enemy", not by God.
Second, there is an option to remove evil from the world, but the result is catastrophic, though not complete, destruction of the world. Take a look at the story of Noah, his family, and the Flood, if you want to see what this might look like. Do notice in the story of the Flood that Noah's family returns to the old status-quo (imperfection) almost immediately after disembarking from the ark.
Getting back to the parable, removing the weeds from the wheat would require identifying them early before their roots are entangled, so as not to uproot both, but the challenge here is that early sprouts may look much the same. Errors in judgement would presumable be frequent.
Alternatively, reapers could wait for the plants to bear fruit, and identify each by its fruit. This would be best done at the harvest when all is ripe, because now the roots are entwined and pulling up one would mean pulling up the other. Problem is, the wheat and the weeds exist in conflict and competition.
The parable says that God has chosen to allow the weeds for now to preserve the wheat. In a sense, God allows for evil and for pain, so as not to destroy what good there is. Difficult to accept? Yes. But then faith is trusting that God, who is omniscient (all-knowing), has a better perspective of this problem than we do. And this makes sense. After all, our perspective comes from maybe 80 years of personal experience and the perspective on one individual, namely "me". God's perspective, even if God weren't omniscient, comes with a great deal more knowledge and experience. So, personally, I think God might be the expert here.
That doesn't make evil any more tolerable. Nor does that make God's decision any easier to bear. Thus Christ says, "Take up your cross and follow me." Life is a mixed bag. And perhaps some fields will have more weeds that wheat or vice versa.
What does give us hope is the end of the parable. God collects all of the field, weeds and wheat, and separates the two. For those who are worried about that separation and its implications, keep in mind that the weeds want to dominate (choke out the wheat) and the wheat wants to grow in peace. No one, however, wishes to be thrown in the fire, but burning was a common method for containing that which is contagious or spreads unchecked.
The point here is that there is consequence and justice. Both what we do and who we are have meaning and purpose. And that is the Christian Hope, the aspiration of the Church - to be what God has intended and do what good and right and pure, as God intended. We want to see God's creation, including ourselves, made whole. We want to be wheat in God's fields, citizens of Heaven even while on earth.
