Because you can't recognize the difference between reality and support. Reality was that Lincoln could not interfere with slavery where it existed, not as an Illinois legislator, not as a Congressman, not as President. Lincoln knew that. He understood that ending slavery would take an amendment to the Constitution and that wasn't in the cards. Notice that Lincoln said 'where it exists'. He knew that if the U.S. could halt the expansion of slavery then it would eventually die. That if the Republicans could overturn the Scott v. Sanford decision then the federal government could keep slavery bottled up where it was and that would hasten it's inevitable demise. That isn't support, that's realizing the limits on what he could do. The number of Lincoln quotes showing his opposition to slavery are numerous and show what he felt.
If a person was a legal slave wouldn’t this limit his/her liberty?
Depends I guess. If you're someone like Lee or Jackson or Davis and you believe that a slaver was property and not a person then you believed they had no liberty and no rights to begin with. If you're someone like Lincoln you realize that they are entitled to those liberties but the institution of slavery is preventing them from enjoying them. And you work to end it.
For the purpose of this post, let's stipulate that you are correct: Lincoln, as early as 1858, was working to end slavery directly, or indirectly.
This is the exactly opposite of what candidate Lincoln was selling to his state and national audiences.
Let's look at his own words: “I will say here, while upon this subject, that I have no purpose, directly or indirectly, to interfere with the institution of slavery in the States where it exists. I believe I have no lawful right to do so, and I have no inclination to do so.”
Note well the words “no inclination to do so.”
It is ironic that you take the southern view at the time that candidate Lincoln was an insincere charlatan.
Congressman Lincoln could have introduced a proposed constitutional amendment to end slavery. He did not.
He felt he had good reasons not to do so: it wasn't in his economic and political best self-interest.