I'm not sure but I thought it might have been to stop folks short cutting the process.
VW did it with their air cooled engines,they stopped fitting a drain plug.
This was to stop people draining the oil without cleaning the filter....

I agree it can be messy,I used a cat litter tray to do mine it helps contain the initial tsunami of fluid.
Funny how even a little drop of ATF on the garage floor can look like a major oil spill disaster!

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.