one of the largest yes, but the latter part of your statement is completely off.
just talking about nvidia AIBS alone off the top of my head, theres: MSI, Gigabyte, Asus, Zotac, Galax, Colorful, Inno3d, PNY, Gainward, Palit.
and this is just nvidias optioins. There’s a lot of competition. EVGA was a favorite to those living in the U.S due to having reletively better customer service, but it was far from not having competition.