Because forcing everyone to pay a chunk more money for on-board storage which not everyone needs is unnecessary. Their decision shows that cost is a more important feature and I don't think that should come as a surprise, there's plenty of stuff out there justifying the various decisions and it all comes down to cost.
I'm guessing if the RPi Foundation went that route with the Pi 5, it would be similar to the Compute Module where there would be a cheaper 'Lite' version without eMMC then more expensive ones with eMMC.
