Because people like burgers and hot dogs? Because changing peoples diets is easier if they have a similar foods they can cook? Because people want to make a positive change, without limiting their options? Because people want to introduce others using safe familiar foods?

There are a myriad of reasons why, and many of them have to do with transitioning people from a meat focused diet to a meat free diet. Usually by changing 1 ingredient in a dish rather than upending their whole process.

Also, there are plenty of vegan meat substitutes that are excellent. Eg. if you are looking for a vegan substitute for meat in stew or pulled pork look for Fable (its just shredded shitake with food binders) https://fablefood.co/ (Not affiliated, but its delicious).