It has to do with resolution. Per solid angle, most of the stars, and even
Venus are brighter than the sky away from the sun. If you can restrict
your resolution so that there is not much sky "surrounding" the star, then
you will be able to see it. Unfortunately, our eyes are frequently not
good enough to see anything during the day other than objects with very
large solid angles (Sun, Moon, Venus). With a telescope, you should be
able to do better, but ultimately the limiting resolution of atmospheric
turbulence will stop most observations.
In a deep canyon (or hole, or well), the light levels tend to be lower.
Eyes will open wider and that increases resolution, so you should be able
to see Venus, and maybe Jupiter and possibly Sirius if the light levels are
suitable. I don't want to do the math to check it though.
Incidentally, I have been told that squinting helps seeing because it
restricts the entering light - causing the pupils to dilate and improving
the available resolution. Is there any truth to this?