Why is the reward decreasing while the difficulty of mining is increasing? I was wondering why Bitcoin has been designed in a way that rewards are decreasing while the difficulty of mining is increasing.

To me this is counter-intuitive and leads to the situation when miners from the early era were able to gain substantially more coins for doing substantially less work. Isn't that unfair?
Early adopters or miners make more profit. One could call this unfair, but that's life. Why does it need to be fair?

The reward is decreasing because the total amount if bitcoins is limited of time and the idea is that the incentive for mining should shift over time from the reward bitcoins gained when creating a block (aka. the coinbase transaction) to the transaction fees of the transaction included in that block.