It's because this is still in its infancy. VW has produced ICE cars for 90 odd years, and in the ICE world, what distinguishes one manufacturer from the other is principally the engine, and to a lesser extent the chassis. The engine is a very complex piece of engineering that allows brands to distinguish themselves. An electric motor is far more simple and my guess is that in 10 years from now motors will be sourced from a limited set of suppliers and car makers will build cars around stock parts for the most part, which includes the motor, the batteries, the BSM and the software to manage it all.
The ID.3 heralds the beginning of a new era of electric mobility for Volkswagen. The electric drive, which is based on the modular electric drive matrix (MEB), is produced by Volkswagen Group Components. It’s not only less complex than a conventional petrol or diesel engine, but also compact...
As said above, electric motors are pretty straightforward and it's security of supply that brings it all in house especially as the market begins to take off.
Like all other car parts, it'll be farmed off to a third party before long.