They don't always do this. Sometimes it's the exact opposite.
Take Gragas for example recently. He received a few nerfs initially and those were fair, but then they reduced his W damage AND nerfed all of his core items in back to back patches. Vi had her Ult damage reduced awhile back and then Warrior was nerfed shortly after. The same thing happened with Sejani's damage and Cinderhulk getting nerfed at the same time earlier in the year.
Sometimes compensation buffs are given to champions when they are nerfed, but other times, indirect changes nerf champions alongside direct nerfs to their kit. This can happen when buffing champions as well, look at Lee Sin recently to see what I mean.
I don't know why they do this all the time, but I figure it is just a lot of things that they have in mind at the time. Like right now, they want to make these juggernauts be a real threat come lategame, but they have to compete with extreme mobility and kiting so when they consider the champion to be too strong in this case, they want to make sure that they don't overdo it and make them completely weak when they nerf them as they have a lot of innate counterplay in them mostly having to walk up to a target to do any real damage.
I don't really agree with it in this case as I think Darius got some buffs that weren't necessary in 5.17, but we'll see what happens.