1) Cost - it isn't cost effective to remove and replace the brake and shift controls, recabling the shifters and likely the brake levers, replacing the stem, replacing the handlebar and adding grips and possibly end extensions...
2) Aerodynamics - wind resistance matters, particularly on a tourer that's used for touring. Across parts of the US, the winds are relentless. The tuck afforded by drop bars is difficult to achieve without some unusual flat bar downward and rearward extensions.
3) Comfort - subjectively, I've never, ever been as comfortable on a flat bar for long periods of time (talking greater than 4-6 hours) than as on a drop-barred bike. Never. On drop bar bikes, longer saddle times are possible (for me). Others will argue this.
4) Geometry - as has been mentioned, most traditional road frames are not designed with flat bars in mind. So, you'll be using the stem to correct for what will put your torso at the right angle and you hands at the optimal reach. This is separate from cost (1), as the answer may take several iterations to solve to your satisfaction.
If you want a flat bar bike, then buy one to begin with.
Last edited by Phil_gretz; 06-11-20 at 06:47 AM.