Pretty simple, because AMD GPU's were largely irrelevant for 10 years.
Outside of the legendary 7000 series released in 2012, AMD GPU's have been mediocre and had a history of heat/driver issues.
The next release was powerful but stupidly hot and loud with the r9 290/x hitting like 90c+ (2013)
300 series was absolute garbage (2015)
next two generations rx 400 and 500 gave up on the high end and focused more on budget gaming. rx580 was a great card for the price no doubt but AMD was basically out of the mainstream GPU game (2016/2017)
Vega cards were a joke, power hungry and competed with NVIDIA's GOAT Pascal Generation (2017)
RDNA 1 is when AMD started to get their shit together, good value for the money but did not compete with Nvidia in the high end (2019)
RDNA 2 is when AMD finally started to compete both in price and performance (2021)