1B aren't as valuable because the position demands the least amount of physical tools.
Scooping/picking throws in the dirt and feeding the pitcher covering the bag don't require a great arm or a ton of speed. Therefore the pool of MLB players who could possibly man the position is extremely high. Now that doesn't mean that just because you meet he minimum physical requirements (which nearly everyone who steps on an MLB diamond meets, save for a few) doesn't mean you'll be good at it. But you can at least try to play the position and odds are with enough reps you might even be able to do it adequately.
Compare that to SS or CF. You WILL have to have a minimum amount of foot speed and arm strength to play those positions. If you don't have those physical tools, you will never be adequate at it no matter how many reps you take or how good you are at the technical aspects of the position, etc.
If you want a real life example, use Christian Walker. He's seemingly the best defensive 1B by far and he's got 25th percentile sprint speed and 5th percentile arm strength. He simply cannot play anywhere else adequately while being THAT slow and with an arm THAT bad. Yet he's still the best defensive 1B in all of MLB.