Because soils in and of themselves are generally productive. Aerating soils (tilling) boosts the microbial community, making it consume organic matter in the soil. Biochar is highly stable, so it wouldn't be consumed, but instead the existing organic matter, which acts as the massive pool nutrients would then be consumed. So what you'd see, is a priming effect at least a few years, followed by a drop in soil productivity.

Biochar is useful as a soil amendment in the right conditions, but its effect is really situational and highly dependent on cooking temperatures and feedstock. I used stuff cooked around 380C in an attempt to allow for some of the feedstock's more available carbon forms to be accessible to microbes. It didn't really work like that, but I ended up with an N boost in my soils, which we suspected was due to proving more microbial habitat (due to char surface area).

It's also really good at catching and holding onto leaching metals, and nitrogen too, in the right conditions