Gravity is weak, but it's cheap. It is indeed surprising, but plenty of research shows that the levelized cost of gravity storage is cheaper than Li-ion batteries - even if you assume that Li-ion will get 4x cheaper in the next 20 years. There are other problems with Li-ion as well that will prevent it from scaling up to the amount of storage that our grid will need. I discuss all this with lots of citations in a white paper here https://github.com/syllable-hq/uphs-feasibility-study