Just of the top of my head here are some possible ideas to explain why not:

• Meat subsidies
• Meat substitutes require more processing and additional ingredients
• Meat sells a lot more than meat substitutes hence the whole chain benefits more from economies of scale
• Animals raised for meat can extract nutrition from plants and parts of plants which humans cannot (for example cattle can actually break up the fiber in food and extract nutrition from it, which humans cannot), plus they can eat plants which are far more hardy than most plants grown for human consumption. Some will also eat other animals which humans do not, such as insects.

I doubt it’s just one of those things that is responsible and suspect it’s a mix of those and maybe more.