Another one that can't understand how it works.
Let's try to be more clear:
1) hashing power increases
2) THEN COMPUTATIONAL POWER REQUIRED TO MINE INCREASES AS WELL
If they cut mining times now by 50%, they find themselves with mining time INCREASED at next DIFFICULTY ADJUSTMENT (this should hint you something) so much that they will not earn more.
Essentially, hashing power increase is just to overthrow competition.