With as good as MM6-8 are (and the whole series for that matter), is there a reason why a decent world builder or clone like these games has not come out nearly 20 years after this awesome formula for games was developed? Are the constraints primarily legal or technical? Is there a fan made clone or something similar to breathe new life into this series?

There are some mods for these games. The Chaos Conspiracy for M&M6 is probably the most known one.

That looks fantastic! I will check it out; thank you for the link!

Are there similar popular "new world" mods for MM7 and/or MM8?