Middle children need as much as any other child in terms of attention, guidance, affection, and even material things. Some parents "forget" this important fact. If one is going to be a parent this detail must be thought through prior to having them and if one cannot provide equal love, attention, affection, perhaps just one or no children would be best.
But no, many parents treat having children like bragging about having various material things and just expect the difficult job of caring for them to "come" to them without thought or effort. For many, the intricacies of parenting never comes to them. Love and affection can be shown in many ways, too many parents "forget" this important fact. Perhaps some parents don't or won't understand that the middle children should be treated just as those who are older or younger are.
For example, when financial circumstances are the exact same if not better and the older and youngest of three are given cars and parents pay for braces for their teeth, why shouldn't the middle child have these things as well?