There are a number of reasons. 1) You won't always have access to the 1.5MW charging. 2) Stopping to charge, even if it's just for a few minutes, is inconvenient. 3) A larger battery will allow for greater lifetime range before it degrades to <80%
You forgot the biggest reason: Winter. What works for benchmarking under nice Cali weather condition doesn't work for IRL. Current generation lithium ion estimates are ~30-50% halving of range in winter (it's especially bad if you don't have your own garage with charging and pre warming the battery can easily reduce the availability battery capacity by another few percent in exchange for better en route efficiency).
Which is why the new sodium-ion batteries are likely to be preferred in cold climates, because even if they cannot reach energy/mass ratios as good as the best lithium-ion batteries, they lose much less of their capacity and charging speed, down to -40 Celsius degrees (-40 Fahrenheit degrees).
While for now sodium-ion batteries have similar prices with lithium-ion batteries, due to being on the market for less than a year, their second advantage is that once their production becomes more mature they should be significantly cheaper than Li-ion batteries.