because meat was a luxury and because the Catholic tradition forbade meat on Fridays, in Lent, and on numerous other days of abstinence. Some specific ones that come to mind:
-Spanish tortillas - nothing at all like Mexican ones, these are more like fritattas, made with eggs and sliced potatoes, sometimes with onions or roast peppers. There's the whole fritatta/omelet family, for that matter
-dough stuffed with stuff: ravioli made with cheese or mushrooms or squash or spinach' cheese or saurkraut pierogi
-noodles with stuff: pasta with tomato sauce or garlic sauce or herb pesto, mac and cheese, kasespaetze, lasagna. Along the same lines there's risotto with mushrooms or squash.
-porridge: at least one Friday a month my mother made a rice/hot milk concoction, sort of like Jook. Polenta is essentially a porridge, and can be used as a base for various toppings
-bread and stuff: pizza with vegetable toppings, flamm (an Alsatian concoction similar to pizza, but with no tomato sauce), crostini, grilled cheese sandwiches, Welsh rarebit
Then there's India, which has a vast array of vegetarian dishes.
One of my favorite meatless stews, especially this time of year, is American. Zuni Stew, from one of the Green's cookbooks, is winter squash, beans, and corn cooked together, with tomatoes, peppers and onions, and served with corn tortillas.