In women's clothes, if you're trying to target Generic Woman, you often have to put deep pockets somewhere other than the front of thigh, because for various compositional reasons that area isn't reliably available across women's various body shapes. This is how we ended up with one of the coolest innovations in pockets since leggings: the side leg pocket. Everyone has a side leg. You can put a pocket there and it's perceptually irrelevant to people. Some of my skirts have pockets designed along these same lines that can hold an entire Nintendo Switch with controllers attached and it's effectively invisible if I'm sitting in a chair, because no one notices when my thighs are a bit thicker on one side. In leggings it's blindingly obvious that there's a phone there, but leggings are typically bodycon-tight to begin with, so I just view it as "yup, that's a phone pocket" when I even remember where I put my phone at all.