Because you have wasted like 5 opportunities that NVIDIA has given you to penetrate the market on a silver platter
The 7000 series could have been so successful if they would have been willing to undercut NVIDIA significantly, it was the perfect time to strike at a time when there was a lot of discontent with NVIDIAS ridiculous pricing and VRAM offerings. But AMD would rather sell two cards at 1000 each than take the bet of trying to sell four at 750.
At least in the past you could argue that they were not in a position to take huge bets in the graphics market but they've been doing too well as a company for that excuse to hold any value anymore. The last two generations can only be explained with complacency and lack of ambition.
At this point AMD kinda deserves to give up their position to Intel in the graphics market, let's see if the new player is willing to penetrate aggressively because the old player two is clearly ok with where they are at.