There are real-world problems with talking to them officially.
Jeff Davis had sent his representatives with papers that could not be accepted. If Lincoln/Seward officially accept those documents and the men who carry them as representatives of this Confederate States of America, they declare secession an accomplished fact with those seven States already out of the Union and officially a sovereign nation. Lincoln and Seward were canny enough to realize that and avoid it.
Davis had his problems, but he was not a dummy. He knew what he was doing before he sent the representatives.
So the response of Lincoln's administration (not accepting the representatives officially, but having Seward meet with them unofficially) is about the best that could be expected or hoped for in the situation.
Seward -- obviously -- had not figured out yet that Seward was not President and that Lincoln was the boss and intended to act like the boss. Lincoln had to haul back on the reins and show Seward who was in charge. I understand the Southern representatives feeling abused, but it was Seward misrepresenting things, not Lincoln. In any case, the Southern representatives barely made a diplomatic effort. I think they were in Washington only a few days. It is only five weeks from Lincoln's inauguration day to the attack on Fort Sumter.
Lincoln felt -- like Buchanan before him -- that the President had no authority to recognize secession as a legal fact. Congress might, but the President could not in their view. With that as a starting point, the duty of the President becomes to maintain the Union first while perhaps facilitating negotiations between the supposedly seceding States and the Congress (no matter where that might lead).
It is certainly a rock-and-a-hard-place situation. Lincoln was trying to gain time while building a consensus. Davis, OTOH, saw that he needed to convince the Upper South to secede if the new Confederacy were to survive and thrive. He attempted persuasion (rhetoric, promises of political favor, vague hints of future consequences). Seeing a need for urgent action, he ordered the attack on Fort Sumter -- and the war began.