Sure I know I can mix my own fines herbes. I also do mix my own pumpkin pie spice. I can also mix my own herbes de Provence (if I can find lavender), or my own Italian seasoning, or my own garam masala, if that's what I want to do.
But the topic of this thread is about is not what I am powerless to concoct. It's about what is or is not in line with contemporary tastes.
The fact is that you can buy pumpkin-pie spice, or Italian seasoning, or garam masala, at very many places. Even premixed everything-bagel seasoning is not so hard to find.
But premixed fines herbes - that used to be much easier to find than it is now.
Besides, "not as good" forms of a food have been with us for at least a century, and they're still with us now. So by itself, "not as good" can't be the sole reason for an item's disappearance.