Because people just don't get it, and Forge doesn't explain it. So I guess we have to.

Inside out common config, we should have a really big comment block that explains (nicely) that machines & ores are now configured on a per world basis. We should instruct people where in their world saves they can find this (including a mention of how it works on servers) and we should explain the defaultconfigs folder.

That should hopefully cut down on being asked about it every week on discord.

Having to change certain config-values for each world is rather tedious, esp. when doing lots of testing and creating lots of test-worlds. E.g. ore spawning. Is there a way to disable/change ore spawning before the world generates? Seems odd to have ores generate around spawn only to remove them again.
Perhaps some options should/could have a global default in the common config?

No, they are per world on purpose and that is exactly what the defaultconfigs folder is for. This isn't a suggestion, it's a ToDo/Reminder.

Ah, TIL there is a defaultconfigs folder. Thank you.