Because suction mounts often fail at the most inopportune times. At altitude, there is less pressure difference than when on the ground. So even with a clean surface and clean suction cup and properly applied, both my iPad and the Stratus 2 have made multiple escape attempts by jumping off of the window onto the floor. Especially after a bounce from turbulence.
For most aircraft, you can't position the tablet with a suction cup to be within your instrument scan as you propose. You can get close with a yoke mount, it it's still in a peripheral view than within the main scan area. Suction cup mounting is often to the side window or windscreen. And then you'll need to be careful of not blocking a key sight line.
Finally, your head is not "down all the time". If it is, you're spending too much time on it versus flying the aircraft. I will spend a few minutes at times, but only when in stable cruise. Other times, I have set it up to give the info I need at a glance, and only then to confirm what I already should know.