JustSniloc had a good answer, but let me expand on that.
First, if you want bigger looking thighs, you need to be targeting your glutes and hamstrings. Why glutes and hamstring and not quads? Well first, a lot of knee problems stem from weak hamstrings and underactive glutes, so you'd be injury proofing yourself as you age. Two, Glutes are the one of the ...
Short answer: Safety and culture. There are no studies proving it.
Most people using a training plan are amateur runners, and many are beginners. They are self-coached. Elites have coaches.
As others have said, for beginner runners, LSD is essential for building endurance, since it is either not possible or not safe for them to run long and fast.