In part, time and money--I don't have the money to spend on throwing parties as often as I used to, and I've somehow lost track of time. (Potlucks aren't ideal, because I'm vegetarian and most other folks would bring meat. So I would usually cook stuff. One year we made individual pizzas on pita bread on the grill for my birthday, which was great because I just bought a bunch of veg toppings (sauce, cheese, onions, bell peppers, mushrooms) and people assembled them. But my birthday is in March so it's a crapshoot if the weather will be good or terrible.)

Another part is that I don't think people like me or want to see me, so I think they'd say no to an invitation regardless. This is reinforced by people not inviting me to things, in a delightful ouroboros.