This is factually incorrect, not an opinion and I can prove it with screenshots.
Reach had better looking skyboxes even graphically, and older games had better looking skyboxes aesthetically (read the part where I say aesthetic before you say something). Halo 5 does not have a single good skybox outside of the one time when you are running down the length of a guardian, which is actually just an image again...apart from that every single skybox lacks detail in the sense that they do not have any striking features, they are almost always faded out and they are static too. I am specifically talking about skybox not background details like ships and gunfire that are closer to you than they are to the sky.
Halo 4 actually just used compressed images for skyboxes and that is the reason why it ends up looking worse than even Halo 3 because they didn't use a compressed and static image for skyboxes. but rather rendered most of the details and as such they could animate them and make it look better. There is nothing in either Halo games by 343 that even comes close to
this skybox from a multiplayer map in Reach and it isn't even the most impressive one in the game.
Also just because a game is older does not automatically make it inferior graphically by the way, keep in mind that Halo 3 despite looking worse (due to other aespects of its graphics) and releasing in 2007 on an Xbox 360, still had a lighting that was technically superior and more accurate to the one seen in every other Halo game released after it. Or how Halo 3 still has better water than even Halo 5.