Why do modern kids argue with parents more?

It's expressing a need for self-determination..

"… The articles in Parents showed that children were increasingly autonomous when it came to their self-expression, particularly in relation to daily activity chores, personal appearance and defiance of parents. In contrast to this increased autonomy that child-centered parenting has given children, the 20th century has seen, in other ways, children's autonomy curtailed, through increasingly restricted freedom of movement and substantially delayed acceptance of responsibilities. Children now have fewer opportunities to conduct themselves in public spaces free from adult supervision than they did in the early and mid-twentieth century.

Dr. Rutherford concludes: "Today's parents face demands that require near-constant surveillance of their children. Allowing children more autonomy to express themselves and their disagreements at home may well be a response to the loss of more substantial forms of children's autonomy to move through and participate in their communities on their own."

See: http://www.eurekalert.org/pub_releases/2009-08/s-akt080409.php

Rutherford MB (2009).Children's Autonomy and responsibility: an analysis of childrearing advice.
Qualitative Sociology
DOI 10.1007/s11133-009-9136-2