part of that, for LA at least, is that the mixed-use here still requires parking, which is expensive. so it's designed with low commercial square footage to make room for ground-level and above ground parking, which is cheaper than subterranean parking. the commercial space is minimized to a token amount likely to meet the necessary zoning/permitting restrictions to get the benefit of higher floor area ratios (FAR) for the least cost. then the businesses that can fit in the space, given the high rents, aren't nearly as attractive to residents, who are also paying high rents.