Can’t I use the Mac finder to preview tbx files? 
No. The image you see in Finder’s Get Info pane is of the View pane at save/close. However, I don’t believe there is a method allowing quicklook to see deeper into the file.
FWIW, ‘preview’ is a slippery term here. In a TBX with a few notes and no agents, action code etc., and using no Displayed Attributes, then previewing a note might be as simple as its title and body text. In reality, many docs are much more complex so you can’t ‘just’ look at a note and see what you see in the app. I assume this is why Finder/quicklook shows the information it does.
If you just need to see the data, simply open the TBX in BBEdit or the like - the TBX is simply a large well-formed XML document so can be read as XML (though the rich text aspects of $Text are stored in a further encoded in Base64 format (see). Inline images in text are stored in the rich text data.
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What would you like Finder to show you?
If you are looking to search inside TBX files FoxTrot Search is AWESOME!! @TomD turned me on to it. Love it. Helps you peer into and search TBX files.
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I didn’t explain it well enough. This is what I meant by the preview icon.
Just paid for the personal version - very impressive - thanks for the reference