Are discussed

Thanks a lot Clive

Sorry for the sentence below that I asked in the link below too, they said "have discussed" is correct. Can you please explain why is "are already discussed" wrong?

"In this chapter, we introduce another sorting algorithm: heapsort. Like merge sort, but unlike insertion sort, heapsort’s running time is O(n lg n). Like insertion sort, but unlike merge sort, heapsort sorts in place: only a constant number of array elements are stored outside the input array at any time. Thus, heapsort combines the better attributes of the two sorting algorithms we have already discussed."

https://forum.wordreference.com/threads/have-already-discussed-discussed-are-already-discussed.3548673/