What causes the earth to be a good grounding system?
The electricity you're talking about is the man-made kind, which is completely independent of any charge the Earth has relative to its atmosphere. Man-made power sources are almost all referenced to the Earth as zero volts, as established by connecting the "ground" to a conductive rod driven into the ground. Each house's electrical system is also grounded in this way. Since earth (lower-case "e") is at least somewhat conductive (except for perhaps dry sand), if you grab a conductor while standing barefoot on the ground then current will flow through you, through the ground, and then back to the source of electricity and/or your house's local grounding rod. Enough volts, and good enough conduction, and you get hurt or killed. Rubber shoes, rubber gloves, and just plain common sense can prevent this.

Again, this has nothing to do with the charge the Earth has relative to its atmosphere, as man-made power is generated referenced to Earth ground; plus, since it's almost always alternating current, the polarity changes 50 to 60 times a second, so any purported existing voltage wouldn't protect you.