And I think the explanation is clear: proponents of mindfulness are generally not willing to apply analytical thinking even in the service of greater mindfulness. Attempts in this direction have been made, but I have not been impressed.
https://www.amazon.ca/Buddhism-without-Beliefs-Contemporary-...
http://slatestarcodex.com/2017/09/20/meditative-states-as-me...
(The particular examples I have are from the Buddhism-specific conception of mindfulness, but I don't think the more general conceptions you allude to have been more successful. I'd be happy to see links to the contrary.)
I highly recommend the following books, which really help you chew through the layers of expectations, belief and doubt when it comes to Buddhism (and in particular, Zen):
http://www.amazon.com/Ending-Pursuit-Happiness-Zen-Guide/dp/...
http://www.amazon.com/Buddhism-without-Beliefs-Stephen-Batch...
An excellent book on this exact topic is Buddhism Without Beliefs by Stephen Batchelor[0]. Very readable & super short. Recommended!
[0]https://www.amazon.com/Buddhism-Without-Beliefs-Contemporary...