Vegetarian fast food

It is interesting that while MacDonalds sells vegetarian burgers in the US, the same item is not available in Singapore. Considering that the majority of people in Singapore are budhists, we should have a sizeable number of people who are vegans. So MacDonalds (and other Quickservice restaurants) are missing out on this large consumer sector. I wonder why?

The additional cost in providing vegetarian burgers cannot be high. The buns, lettuce, tomatoes and french fries can be served up as is (with sauce if required) and it can come with vegetarian patties, if desired. And all QSRs today claim to use vegetable oil, so there's no issue about animal ingredients involved in the preparation.

So the additional cost of providing the vege menu cannot be significant. Why isn't vegetable burgers offered in Singapore, or is it because the vegans generally shy away from fastfood anyway? I'm not a vegan, so I don't know.