Because batteries are heavy. As you add more batteries you have to add extra batteries to help push the extra weight which adds even more weight and costs. And as you add more weight to push, the efficiency of the system gets worse.
In my 10 year old Chevy Volt, I can regularly get 4-5 miles per kw. It only has a small battery pack though and I only get 40 miles before I have to use gasoline.
However, in the much newer all electric cars with giant battery packs, some only get 2.5-3.5 miles per kwh because they have so much more weight to push around and unlike IC engines, the car doesn’t get lighter as the fuel runs out.
For your use case, you might want to look in to hybrids. In my chevy volt, I just went 6 months without needing to fill my gas tank because I didn’t really go anywhere other than to work and back. But this past weekend, I had to drive about 350 miles so I used gas and got 35mpg. The new phv prius is the spiritual successor to the volt.
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