Because it doesn't make a lot of sense to invest heavily into saving 100kg on the frame and then adding a 500kg battery. The i3 had a small, lighter battery and an extender.
The range killer is really drag, not weight. Weight contributes, but at high speed drag is such a large draw that nearly nothing else matters.
The rolling resistance coefficent of a car tire is about 0.01 and the force grows linearly with mass. Drag is v^2 and the coefficients are more like 0.2 - 0.3 of the frontal area on most EVs.
Weight savings don't offer that much range savings so there isn't much incentive to pull weight out of a design, especially when carbon fibre tub construction is so much more expensive.
BMW made a bet batteries would remain very scarce and expensive, a bet they lost pretty throughly.
