A crack developing there, is most likely too much seat post out of the frame, or in actual fact, not enough seat post being in the frame.

The biggest problem with a crack in this location, is that it then causes most force to be placed on the join between the top tube and seat tube. You also have to think about what would happen if the frame were to suffer a critical failure in this position - it will most likely be landing from a drop or, more likely, while sitting and hitting a hole/bump; neither great options and the more likely of the two being much more painful!

Drilling a crack is to minimise crack propagation. And there are few repair options with this style of frame (for sensible money).

To answer the question in the title - aluminium work hardens quicker than other materials, and tends to crack due to fatigue much quicker than say carbon, steel or titanium.

Unfortunately I think it could be time to consider a new frame.