Mstram,
Like cargo rat, we ensure people are competent at manual w&b before allowing them to produce EDP loadsheets etc.
You would be surprised how difficult it is to exceed limits (cargo aircraft excepted) actually.
Passengers have an influence that must be considered first and then you control the CofG with your deadload.
Simply, wt x distance = a moment.
That moment is usually converted to a much smaller number by using a constant denominator (10,000 30,000 100,000 etc) to give a index value for ease of use.
Don't forget that on large aircraft you will load both ends, so the 'net' effect is not that large.
Some aircaft are 'tricky' with low passenger lows (MD80 B727 etc) where they become extremely tail heavy. Combi aircraft present unique problems with compartment weight restrictions dependant upon other compartment loads, lateral restrictions etc.
However, W&B is not that difficult if you give it the time and thought necessary. In fact weight problems occur more than trim problems. Sounds difficult to control the CofG within such a small linear dimension; but in real life it is not such a task.