Why not reflow a PCB before placing parts?
Because the method you describe, and it's advantage, is really a disadvantage. Not only is it harder to place smd parts on unevenly tinned pads, it prevents the main advantages of solder paste.

The first is that solder paste holds a piece in place, ever so slightly, before being reflowed. It is a paste of tiny solder beads and flux. The smd parts are suspended in place.

The second being surface tension. When freshly reflowed, free of oxidation and contaminants, there is high surface tension. The parts are not likely to move without external force. Second and third reflows result in lower surface tensions, as well as cold joints.

And the last advantage is capillary action. Between the flux, the surface to surface contact of the pad and the pin, capillary action pulls the pin to the center of the liquid solder. Multiple pins doing this, makes a smd part self center. And as long as you have solder mask between the pins, this is even more effective, of course, only if there is not excessive paste on the pads.

And most importantly, industry leaders have probably tried your method, and having not adopted it, it probably didn't sit well with them. Probably.