Because it would be a lot more than $2k to get put it in your home. If you do the math on a Tesla powerwall, the raw cost of the battery should be right around $2k too. But it actually costs more like $8k per powerwall.
Competition is low. There are only a few players in the home battery space and they are similarly priced.
Demand is high. Even with the higher price, the wait list to get a powerwall is more than a year out.
My expectation is a few things happen over the next 9 years.
I expect that battery production ramps up to a high degree.
I expect that the EV market will eventually be saturated
With those two things, I expect that EV manufactures and the likes of tesla will end up with more batteries than than demand. When that happens, you can expect that the cost of battery products will start to tumble as everyone that manufactures batteries starts to try and sell them for whatever application will buy them.
Things that could spoil my assumptions?
Power companies might decide now is the time to really buy batteries. That could keep demand outpacing supply.
We could see new applications of batteries that aren't currently on the table, such as battery powered trains.
The EV market could not saturate. This could be due to battery production capacity severely lagging EV demand. This could be due to anything from manufacturing plants running into major issues to supply chain problems.
But, I'm an optimist. I expect to see battery tech and manufacturing continuing to build out at a fast pace.
Oh, and eventually the expensive part of a battery system won't be the batteries but rather the install cost.
Batteries could get a train up to cruising speed requiring a smaller train engine and less fuel.
Batteries could save on line expansion, instead of installing a 3rd rail for the whole line, they could go a 3rd rail every 50 miles for 10 miles.
I agree a 3rd rail would be cheaper in the long run. Batteries could help bridge the gap.
Some major cities in the US had overhead powered "trolleys". But, car lobbies destroyed those :(.