I’m not completely sure why

I think it’s marketing

5000 mAh is much a bigger number than 19 Wh and marketing loves huge numbers

Kinda like BMW did with the i3.

In 2013 Tesla was selling a model with a 60 kWh battery so BMW had the genius idea to install a 20 kWh battery BUT refer to it as “60 Ah” battery.

Tesla introduced the 90 kWh battery? BMW responds with a 94 Ah battery (28 kWh)

Newest Tesla has 100 kWh battery now? BMW has 120 Ah battery (38 kWh)

“See? Higher number!”, says the marketing

And in order to have a comparable range number they had to implement heavy weight reduction techniques like using carbon fiber for the body, negating any cost saving from the smaller battery AND giving the owner a total loss after small collisions as it shatters instead of bending