There are two finite resources we are talking about here.
- Players
- Camps
For every PL-party there is at least 2 players less that is available for grind-parties. And for every PL-party there is 1 less camp that is available for grind-parties. Unfortunately it's a zero-sum game. The existence PL-parties hinders grind-parties. The more prevalent PL-parties are, the more they hinder grind-parties.
Now this logic also holds true for soloing. So by my logic, soloing also hinders grind-parties, no?? Yes ... but to a much smaller extent. It only takes one player out of the potential grind-party pool, instead of two. Multiply that difference to a server scale and it's dramatic. Also, to DarkstarPoet's point, the XP earned by playing solo is arithmetically comparable (half, double, triple, etc) with grind-party XP. Meanwhile PL-party XP is exponentially larger (10x, 100x, etc) than grind-party XP. That exaggerated XP growth only encourages more PL-parties, and diminishes the ability of grind-parties to form even further.
Some can argue that PL-parties are just a "playstyle", and that players should be able to do it because it doesn't hurt anything ... but I wouldn't make that argument.