is the only graphics that makes the game good, nothing more.
The graphics are arguably one of the areas of lesser quality within Halo. Although Halo 3 had some stunning displays, overall the graphical quality was lacking. Reach upped the ante, but still hasn't arisen to the level of quality you'd have expected for its time.
the gameplay is not original, the pysicks of the game either
The industry still regards Halo: Combat Evolved as having revolutionised console shooters. Whether or not you agree with this is up to you, though its influence cannot be denied. It also popularised shooters on the console, and has found a formula which has worked wonders for it. It is unlikely to change.
I do not understand why others like it so much.
Because of the tight, responsive gameplay, the level design, the lore surrounding it, the competitive and tactical depth, the creativity it promotes, the abundance of modes and extras...I could probably go on, though I feel my point has been made. There is plenty to enjoy.