I don’t understand why everyone acts like the civil war was fought to make slavery illegal.

They don't, because the Civil War was fought to keep slavery legal.

That is why the South seceded.

Every single declaration of secession of every confederate state mentions slavery if not at the very beginning than almost at the very beginning.

https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states

I don't know why you think they seceded, but it was undeniably due to the desire to maintain chattel slavery based on their own words.

Oh, hey, aren't you the person that challenged me to read more? Incidentally, I have some reading for you that might shed light on your opening line. It's called the Cornerstone Speech, and it was made by the first Vice President of the Confederacy at what was, functionally, their first state of the union address.

https://www.battlefields.org/learn/primary-sources/cornerstone-speech

But to make it easy for you, here's a juicy quote. It's a little long, but I made it that way to make it clear it's not out of context.

But not to be tedious in enumerating the numerous changes for the better, allow me to allude to one other though last, not least. The new constitution has put at rest, forever, all the agitating questions relating to our peculiar institution African slavery as it exists amongst us the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. This idea, though not incorporated in the constitution, was the prevailing idea at that time. The constitution, it is true, secured every essential guarantee to the institution while it should last, and hence no argument can be justly urged against the constitutional guarantees thus secured, because of the common sentiment of the day. Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the "storm came and the wind blew." Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner-stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth. This truth has been slow in the process of its development, like all other truths in the various departments of science. It has been so even amongst us. Many who hear me, perhaps, can recollect well, that this truth was not generally admitted, even within their day. The errors of the past generation still clung to many as late as twenty years ago. Those at the North, who still cling to these errors, with a zeal above knowledge, we justly denominate fanatics. All fanaticism springs from an aberration of the mind from a defect in reasoning. It is a species of insanity. One of the most striking characteristics of insanity, in many instances, is forming correct conclusions from fancied or erroneous premises; so with the anti-slavery fanatics. Their conclusions are right if their premises were. They assume that the negro is equal, and hence conclude that he is entitled to equal privileges and rights with the white man. If their premises were correct, their conclusions would be logical and just but their premise being wrong, their whole argument fails.

So, maybe that's why some people think the war was about preserving slavery, because that's what the Confederacy said it was about with their own words.