why any circuit connected to the ground doesn't 'leak' all its charge to the ground.
Actually, it DOES. If you are talking about
static charge. (The kind of charge you get in dry weather from walking in leather shoes over a wool carpet, etc.) That kind of "charge" is undesirable. It can cause electric shocks which are annoying and startling to humans, and actually FATAL to solid-state electronics which is not properly protected against ESD (electro-static discharge).
There are diagrams of the neutral wire of domestic AC supplies, of non-floating bench supplies, consumer electronics, etc all with a ground connected to their neutral/negative terminal. I don't understand why that doesn't provide an easier path for the current to flow to than whatever the return is.
Draw out the circuit. In your mind, or even on paper if that helps visualize it better. A simple circuit (a battery, switch, resistor, LED) has a complete path for the current. Whether you connect that part of the circuit to "ground" makes no difference because the path THROUGH the circuit is billions of times LOWER IMPEDANCE than any possible current path through "ground" to the crust of the planet.
Is it because the ground can be considered charge 'neutral?'
"Ground" is considered "reference" or "neutral", or "return"
by definition, and for very practical reasons.
But then why ground at all?
There are several major reasons to "ground". (Or "earth" as is more common usage in BrEnglish).
In many cases, earth/ground is actually used as the RETURN CONDUCTOR for power. Not in your house, but for longer-distance distribution of utility power down your street, for example. In places where utility power wires are strung along power poles in the air, if you look very carefully, you will see that the high voltage wires are strung through the air, but where they tap power off to send to your house, and your neighbors' houses, they use a step-down transformer which has one big, insulated input pole connected to the aerial wire. But the OTHER side of the circuit is a wire that travels down the pole to a ground rod. The actual crust of the earth forms the return path of the utility power to your neighborhood. This is the scheme in many parts of the planet, but not everywhere.
In some places 3-phase power is distributed to users of heavier currents which is not ground-referenced. But after the transformer(s), the power that is distributed through the house or factory, etc. is grounded locally for safety. You want to provide a path for fault-current that is lower-impedance than YOU. Because if YOU are the lowest-impedance path for the fault current, it could very possibly KILL you.
Why not, for example, wire the metallic case of a computer to an ungrounded neutral wire? I.e. it clearly provides more than a 'zero volt' reference,
Yes, that would work, electrically. You don't NEED to connect any part of that current loop to ground to get the power to the computer.
it is a safety feature where current does flow when something goes wrong,?
Yes, exactly.
so why doesn't current flow there normally
Because the desired path for the power is a MUCH LOWER IMPEDANCE than the path to "ground". Current (like most of us) will "take the path of least resistance".