What the others said.
Some of them are also homebrew armies of various people of note. I want to say there were some white dwarf staff that had homebrew chapters that are semi canon? I remember a contest for people to invent a chapter, and GW would pick their favourite to be made canon. I think the result was Legion of the Damned, which iirc even had their own minis as a result?
Not to be a broken record, but there was a bigger focus on this back in the day. 1000 chapters of 1000 marines. Early starter sets used to feature black templars, crimson fists, dark angels, blood angels, and other chapters and sub-factions. The stars of Dawn of War were Blood Ravens. Not like today where the Ultramarines do everything. And back in the day, this was a few pages of a codex. Some example paint schemes and lore ideas for how to make "your guys". Now in the age of fan wikis, everything has to be catalogued and memorized.
"Oh, you never heard of the Obscurum Kiwis Chapter? Get the fuck out tourist!"
It's not just Space Marines. There used to be many guard legions but now everything is Cadia and Krieg. Mordian, Catachan, Vostroyan, Praetorian. Steel Legion being semi-retconned out of Armageddon to make room for more Krieg is considered to be a betrayal in part because of this.
Plus it's just fun. There are some neat ideas that are fun to play around with.