But here's the thing: it wasn't truly Richard's

Philip II would have agreed with you; Richard, not so much. Taking control of Plantagenet lands was much of Philip’s life's work, and he mostly succeeded after Richard died, ejecting king John from all but a rich sliver of Aquitaine. In doing so he asserted long dormant rights as liege lord. It wasn’t just Philip’s project, it was a long term project of the Capetian dynasty.

When Hugh Capet came to the throne in 987, inaugurating the Capetians, royal power had been in decline for more than a century. Each region had it's own ruling dynasty often going back many generations, and while this array of counts and dukes all technically owed allegiance to the king, there were few means available for him to enforce his will outside his own personal domain. Hugh Capet himself had been Count of Paris before becoming king. The Ile de France, a non-huge area surrounding Paris, was his domain as monarch. He had exactly as many resources as his own region could provide. This tended to level the playing field. If a Capetian king chose to say “hey, that land is mine”, the reply from his vassal would be some medieval French version of “yeah? Come and take it.” An aggressive count in conflict with the king could often bring as much or more to the fight as the king himself. If two or more vassals combined their strength, the king could easily find himself outmatched. Capetian kings operated in this environment for centuries, playing vassals off each other, sometimes allying with the count of Anjou agains the duke of Normandy, sometimes allying with Normandy against Anjou, etc.

In 1137, Louis VI arranged the marriage of his son, Louis VII, to Eleanor of Aquitaine, heir to Duke William X, bringing that vast region under royal control. The marriage produced two daughters but no sons. This, combined with a growing estrangement between Louis and Eleanor, led to Louis seeking and acquiring a papal annulment of their marriage. Though this removed Aquitaine from direct royal control, Louis likely expected, as Eleanor’s liege lord, to have control over her future marriage. The next duke of Aquitaine would be his man. Really, Louis was a bit of a schmuck. Eleanor had no more intention of letting him choose her next husband than she’d had in staying married to him. As soon as she reached Poitiers she was met by, and married to, Henry Plantagenet, ten years her junior, count of Anjou, Maine, Tourane, and recently duke of Normandy. Normandy and Anjou, eternal rivals, united under one dynasty, now joined to Aquitaine in defiance of the King. Shortly thereafter Henry becomes king of England. Wealthier by far than the king of France. Disaster for the Capetians.

And Henry II? Of course he wanted all of his French territory to be free of obligation to the French king. But French royal power had managed to extend itself to other parts of France. The monarchy had enough strength behind it to hold it’s own, if not yet to conquer. Henry faced the same challenges as his rivals. Armies were not large in the middle ages, money was always short, committing resources to battle risked a great deal on the vagaries of war, so military conflict tended to focus on the taking and holding of castles in strategically important areas, such as the Vexin.

This is the reality that Richard I and Philip II grew up in. The conflict was ancient, recently intensified, and never settled in Richard’s lifetime. It was a stand-off that would continue until someone could get the upper hand. Richard was a match for Philip. John… wasn’t. But that’s another story.