It's is simply the law of diminishing returns. Given the same technology, the weight and cost won't yield significant range. Tesla has always said, technical advances will allow for added range. The electric car is not new, the early models (no I wasn't there) as shown on Leno's Garage used lead acid batteries. The weight of the batteries limited the range of the vehicle causing their lost popularity to Gasoline. Even ICE manufacturer's calculate the size of the gas tank for a given range of approximately 300-350 miles highway. It is the acceptable "sweet spot" for range vs. weight. Same the Air Planes, the altitude on long-range planes is limited until they burn off the excess fuel. On shorter range (STL's), they place enough fuel to get to the destination and a safety margin. Filling tanks to the max will make flying less efficient . Same with current technology batteries -- added weight diminishes range per added unit.
A battery in the Frunk would also change the weight distribution of the car, affect handling as well as reduce the front "crumple zone". The last thing you would want is a litheon ion battery crushed in an accident --- they don't like heat and pressure. Also, keep in mind the battery is cooled, so placing a "small battery" up front would need additional cooling capacity along with added collision protection -- i.e. another titanium cover, if even feasible.