Melt the butter in a large pot over medium heat. Add the garlic, dried oregano, dried basil, onion powder, and a pinch of red pepper flakes and cook, stirring, until the garlic is fragrant, about 2–3 minutes.
Pour in 4 cups of water and bring to a boil over high heat. Season with 1 teaspoon kosher salt, then add the penne and cook, stirring occasionally, until the pasta is mostly cooked, about 8 minutes (do not drain).
Reduce heat to medium-low and stir in the milk, heavy cream, and cream cheese. Cook, stirring, until the cream cheese has melted and the pasta is al dente, about 4–5 minutes.
Remove the pot from the heat and stir in the grated Parmesan until the sauce is smooth and creamy. Stir in the chopped parsley and adjust seasoning with salt and black pepper to taste.
Make the arugula salad by whisking together the olive oil, lemon juice, finely chopped shallot, and a pinch each of red pepper flakes, salt, and black pepper in a small bowl. Add the baby arugula and mixed herbs and toss to coat.
Spoon the creamy penne into bowls, top with the dressed arugula and extra Parmesan if desired, and serve immediately.