Because ACC's content is boring to be personally. I do not like to be limited to just current GT3 and GT4 cars. I never bothered to download modded cars for AC as I feel like most of them aren't even good but track mods are amazing and I have tons of fun with just the original Kunos cars on different tracks.
I also love the selection of original cars that Kunos gave us, we had vintage and modern GT cars and timeless classics like Mclaren F1 GTRs, Porsche GT1, Mazda 787b and then when you are feeling for something different, hop on the Lotus 98T which is a good representation of legendary cars in 80s F1 like the Mclaren MP4/4 which is rarely seen licensed in any good Sim.
Also, AC has a laser-scanned Nordscheleife, no sim is complete without the Nordschleife imo.