Now there's a difference between cooking and canning.

You can safely make sweetened condensed milk in the home kitchen. What you can't do is safely preserve it.

Safe: I want to make a key lime pie tonight, I've got my limes, I've got a pie crust, I've got my eggs...no sweetened condensed milk. Grocery store is closed too. Okay, but I've got milk and sugar, so I'll just mix some sugar and milk in a saucepan and simmer it down, then I'll use that to make my pie tonight.

Unsafe: I'm going to make a big ol' batch of sweetened condensed milk, pour it into mason jars, boil 'em for ten minutes and put them in the cupboard so that it's ready whenever I want a key lime pie.

There's LOTS of stuff that's safe to cook and eat immediately that's not safe to put in jars on the shelf.