Why is it hard to predict earthquakes?
The easiest way I can explain it is you haven thousands of different material joined in billions of ways with chunks of all different sizes, there are just too many varables for an accurate prediction. becuase earthquakes are caused by a slow build up of pressure it is impossible to predict exactly when it will break because the forces are not spread evenly and the material properties of the rock are not consistent.   

look at material testing, if you have a uniform object under steady predictable force they can predict when it will break with some reliability usually within a few seconds. but across a geologic timescale that kind of accuracy would give you plus or minus thousands of years, and that is under perfect conditions, Add in the complexity of the plates make up and you get even more error. Worse with plates the force on them is not uniform there are several forces acting on it and they are not spread evenly.  

to use your glass analogy, imagine a sheet of glass several yards in diameter, but this glass was poorly made its lumpy so the thickness varies by several inches and is full of bubble and impurities. now try to predict when it will break based on a a group of children randomly throwing pennies onto it. You know it will break eventually but predicting when and where is functionally impossible.