No need for the roots to go deep for water.

Because almost all plants hate WET feet. You want the roots to reach DOWN for water. For almost all veggies, slow deep waterings spaced out is far more superior to a little water every day.

If you water too much, the roots reach the WET perchable water table and don't want to go any further. Root rot ensues and your plants suffer.

Kevin

It's probably more a matter of oxygen than water. Plant roots tend to grow best where there is ample oxygen available within the soil pores. That's always going to be near the surface, especially in water saturated soils.

Oxygen is absolutely essential to plant roots, something that we should always remember.

In sandy soil especially, frequent light waterings produce shallow-rooted plants with no ability to withstand drought. In heavy soils it might create root rot and other saturation-related problems.

It is a matter of necessity and convenience.
When plants can find water and nutrients right in front of their nose , why bother to dig in ? That's smart !. In nature everything pick the path of least resistance ( = easy route) .