Why is GML bad at drawing primitives

draw_set_colour(colour);
//draw_circle(x,y,radius,false);
draw_primitive_begin(pr_trianglelist);
var pointX = x + radius*dcos(angle);
var pointY = y + radius*dsin(angle);
var startPointX=pointX;
var startPointY=pointY;
draw_vertex(pointX,pointY);
draw_vertex(x,y);
for(var i = 0; i<points+1; i++){
    a = angle + i*angleAdd;
    pointX = x + radius*dcos(angle);
    pointY = y + radius*dsin(angle);
    draw_vertex(pointX,pointY); //3rd of previous triangle
    draw_vertex(pointX,pointY);
    draw_vertex(x,y); //2nd of next triangle
}
draw_vertex(startPointX,startPointY); //end final triangle
draw_primitive_end();