Because people want to keep eating things like burgers, hotdogs, tacos, etc. And not everyone knows how to replace the "meat" part. Basically not everyone knows

Because people want to keep eating things like burgers, hotdogs, tacos, etc. And not everyone knows how to replace the "meat" part. Basically not everyone knows or has the patience to build things from scratch. I'm fortunate that my family has time to cook things from scratch but I do see the market for all those replacements, some of them are good some others are yes weird.