For a couple of good reasons.

1) Many of the people who describe themselves as "neoconservatives" have never
been liberals.

2) Other dictionaries disagree with MW.


Nothing gets the rocks off like a fantasy that liberals are going over to your
side in droves, I know, but that's not what the term
means in US politics.

In fact, why
don't we use the definition that m-w.com has for neoconservative just to
make it easier for everyone to stay on the same page.