What's the approximate stack and reach for a 5'10" 178cm man
The lack of definitive answers you've noticed is due to the amount of (often unknown) variables at play here. Body height and inseam are indeed good starting points, but people around the internet won't be able to estimate just how flexible you happen to be. Arm length, discipline and your personal preference on how 'aggressive' of a riding position you want also play a major role.
In this light, the advice you've received is reasonably good. After riding a certain bicycle long enough to get a feel of it, you'll be able compare the S&amp;R of that bike to another one and make a somewhat educated guess on how that other bike would feel for you. The perk of going to a bike shop is the opportunity to throw your leg over bikes of your choosing, which likewise will give you some sort of an idea of which size would fit you.
Of course, your question is valid in a sense that for most people, there exists a ballpark of S&amp;R measurements where they should probably start looking at. However, bike manufacturers often got your back on this. Along with the geometry charts, most of them provide you with a model-specific rider height recommendation. While it's not technically a S&amp;R recommendation for your specific dimensions, it's the starting point you should be looking at instead of any arbitrary stack and reach that you haven't tried yourself.