In many cases parents may feel that their judgement is better than the child's in this matter. The parents have, of course, a lot more life experience, plus they know their offspring well enough to recognize strengths and weaknesses. Young people, lacking their parents' experience, sometimes wish for careers that are not realistic choices. Consequently parents may try to push the child toward a career that they feel is a good match with the child's strengths, in the hopes that the child will be successful in that field.
Some parents may be trying to look out for themselves a bit. They may push a child toward a career with a lot of power or earning potential in the hopes that this will provide the child with the means to keep his or her parents in comfortable circumstances in their old age. A parent who owns a business may try to push a child into taking over the business in order to keep it in the family.
There are also some parents who try to steer their child's career choices based on their own lives and regrets. Parents who dislike their own jobs may try to discourage their children from going into the same field. A parent who did not get to have the career he or she really wanted may push a child toward that very career, perhaps as a way to engage in it vicariously through the child.