Because he's a Miller, and people are scare of Ashley Miller (the Papa of the Miller psycho kids XD)
"Rich and powerful" doesn't make them bulletproof. I mean, someone was insane enough to shoot at Trump. If it wasnt for the insane timing of his head tilt he would be dead by now. Not to mention all they had to do was recruit a dominant omega to incapacitate Grayson to cleanly aim that barrel on his forehead point blank. And considering he goes around doing his thing without guards and the fact that he doesn't seem to understand that he's in danger, he's actually an easier prey than Chase.
You missed the "and people are scared of Ashley Miller" part, not that he's bulletproof -_-
I don't get how no one has ever put a bullet into Grayson's skull.