Paint in a milk jug

I am slowly repainting the inside of the house. Dug out all the old paint we had from the last time which was about 20 years ago. When I open those old cans the lid is rusted on. Impossible to get it off without destroying it, and of course cannot close it after.
I poured the remaining paint into a milk jug and that was way better than the original metal can. It will never rust, can see the colour of the paint, and pouring it out is simple and clean.
Why isn't all paint sold this way? Those old round paint cans have got to go.