Why invest in mining hardware if the difficulty continues to climb at an exponential rate?
The mining difficulty is going up very fast. You are correct. But the mining industry is also stimulating the Bitcoin economy significantly. See this news story, for instance: KnCminer sells $3M of bitcoin mining equipment in 4 days 

My best answer to your chart is another chart: 



From: https://blockchain.info/charts/miners-revenue 

Even with the bitcoin difficulty skyrocketing and the hash rate of the network steadily climbing, the math says it all. 


  (number of bitcoins mined per day + transaction fees) * market price


Now mining at that scale is not for everyone, that is where hashing data centers come in to the picture.  If someone wants to mine some of this currency but cannot afford to buy the expensive gear that is required to keep pace with all of these numbers, cloud services will allow little guys to play ball at whatever prices they can afford to pay.