Well, it's simply down to the way wood glues work. They create a chemical bond, rather than a mechanical one, which results in an extremely strong bond inspite of the fact the glue itself is relatively weak.
As such, you want the tightest wood to wood contact for the best joint, and you want freshly exposed surfaces. The best way to acomplish this is to use a freshly planed or scraped surface. It is a longstanding woodworking myth that a slightly roughened surface will glue better, and this idea is based on a fundamental misunderstanding or how glues work.
The roughened surfaces is beneficial when working with adhesives, which are a different thing to glues, and rely on the mechanical strength of the adhesive to make a strong joint. Epoxy is an adhesive, ergo roughing the surfaces is beneficial. Titebond is glue, so a perfectly flat, smooth surface is best.
This isn't to say you don't get good results by sanding with 180 grit, simply that you don't need to do the extra step of sanding unles it's already part of your process for getting a flat glueing face. I don't sand my glue joints if I can avoid it, and if I have to, I follow up with a scraper to smooth the surface and remove and sanding debris and broken fibres.