Thanks for this question! All of the Abraham Lincoln papers that do not already have transcriptions are part of the By the People crowdsourcing project. For example, the letter from General McClellan you link to can be found here: https://crowd.loc.gov/campaigns/letters-to-lincoln/1861-secession-inauguration-start-of-civil-war/mal1107100/? You'll see that it currently needs review, so I hope you might jump in to push it along to complete!
We agree that it would be fantastic to have a link to the documents that are currently available for crowdsourcing on loc.gov so that researchers like yourself could more directly find them and contribute. So many historical researchers are already transcribers just by the nature of their work. We would love to make it easier for them to contribute that effort back to the digital collections!
There is currently a lot of work happening to improve the functionality and user-friendliness of loc.gov but we are still a resource-constrained federal agency so our team's requests for features (like linking to crowd.loc.gov) are weighed against other requests from across the Library of Congress. Linking to the crowdsourcing project is not currently at the top of the priorities list, but we hope it will get there eventually and this will be a loc.gov feature in the future!