when is the Ethereum Merge

So when is the Ethereum merger? The short answer is between September 13th and he September 15th.

But why don't any sources give a specific date or time?

The reason is that the merge is programmed to be active. It is scheduled in "blockchain time", not real time. Instead of a human flipping a switch to enable merging, it is triggered when the blockchain performs a certain amount of computational work. The technical term is Terminal Total Difficulty or TTD.

When the TTD reaches 587500000000000000000000, a merge is triggered and the network officially switches to Proof-of-Stake (PoS). Mining will stop working on Ethereum. Because overall difficulty is directly affected by network throughput, and network throughput varies, we can only give a rough estimate of when this TTD value will be reached.

Twitter user @reir created a useful TTD tracker at totaldifficulty.com. This tracker allows you to monitor overall difficulty as the merge trigger approaches.