I'm not sure if the "inserts" are really harder to find in new bricks. It could simply a long sequence of "bad luck"... I'm ordering bricks of 5$ since one and half year, and it occured at the beginning to have 28 consecutive orders (one brick at time) without any repacement whereas everybody in the forum was finding HNH, HNR and HNZ replacement almost at will!
However, if you speak about BABN replacements, I agree, but it's not new, because they replace notes indivudually in each brick. You have to sift through many many bricks to find only one replacement. The CNM replace notes by sheet (at least 45 times more), plus the fact the CBN have more defects notes than the BABN. When you find replacement, it's not rare that there is more than one note at time...
So keep searching...