Vegetarian/veganism is not a fashion choice, it's a moral one. It's like asking why a Muslim prepares a dish that's traditionally prepared with pork but instead substitutes lamb. I'm not sure what more explanation you require.
You can't just replace ingredients. Make a lamb dish if lamb is what you want!
Huh? Yes you can. Replacing ingredients, adding ingredients, removing ingredients is the basis for all food culture. Recipes get passed down through generations and those variances over time are what give us distinct and unique dishes.
"Yes, the thing with BBQ sauce and pineapple is tasty, but it's not a pizza!"