why is that?
In many places recycling is still partiality sorted by hand. If your container opened up it would scatter hundreds of razor blades onto a conveyer belt line, where people are picking out pieces that machine sorting left out.
Also, if you are using an aluminum can you may as well throw it in the trash because that what the sorting facility is going to do when they get an aluminum can full of steal. As ferrous and non ferrous metals are processed separately.