The quick answer is because it TASTES SO GOOD! Homemade vinaigrettes have so much more flavour, and you have complete control of the quality of the ingredients, which means you can tailor them to suit your tastes - don't like garlic, then leave it out, don't like parsley, use mint or another soft herb instead. They are preservative and chemical free, therefore so much better for you. Well worth the few minutes it will take to blitz it up in your blender.