The answer is money, but it’s not material cost that’s driving these crappy thermal interface pads, but labor expenses (and I’d guess consistency too). Pick-and-place is absurdly fast at putting components onto a PCB, and if they can put the pre-cut pads onto the board that’s huge for a manufacturer.
It’s the difference between slapping a post-it note, or the dot/line/X/cross/etc method with grease. No contest that TIM pads win for them, any fallouts get handled via warranty.
Bite mark forensics has entered the chat
Anything that can convince a median intelligence jury, will be used by unscrupulous prosecutors and cops.