Because there isn't much money to be made. Frequency control used to be provided inherently in every generator. With renewables that isn't the case and no consumer wants to see a new line item on their bill for all the extra cost of accommodating energy markets and renewables so it is all hidden in the network connection fee which is controlled by regulators. So the return on providing grid services that aren't MWH (energy) is Essentially fixed to what regulators will allow, which isn't that attractive, so better for tesla to make the lump sum profit up front on the equipment sale and let the utility run it for 5 years until the batteries are worn out and the utility have collected their ROI.

In terms of actually bidding MWH in to the market when supply is tight so the price is high, it will end up lowering the price by increasing supply, and the intervals of high prices are few. By virtue of existing the system will reduce frequency of high price intervals since the market participants will know that capacity will come on at some price so if they try to bid really high their is will not be accepted. So batteries at this scale is nice for the system operator and politicians to put a lid on high price intervals but won't actually sell enough MWH back to the grid to amount to much.