There are a lot of things wrong with your line of thinking. I’ve literally never heard someone call the squat and deadlift “opposites” before reading this thread. But if you’re wondering why some people do those 2 lifts exclusively for their lower body, it is because its possible to build big, strong, good-looking legs from doing only those 2 exercises.
As to your point about squats and deads not stimulating the hamstrings or stimulating them only as stabilizers, this is patently wrong. The hamstrings are prime movers in both exercises. You don’t seem to realize that the hamstrings cross the hip as well as the knee, and are responsible for both knee flexion AND hip extension.
Since squat and deads involve moving from a position of hip flexion to hip extension, the hamstrings are prime movers in both cases. Because the loading potential for hip extension exercises is so much greater than that of knee flexion exercises, it is possible to achieve far greater stimulation and development of the hamstrings by squatting and deadlifting than by doing leg curls.
Finally, your ideas about anterior pelvic tilt are a bit off the mark. While it is true that the hamstring muscles are lengthened and/or inhibited in cases of true APT, this is usually a product of the pelvic positioning rather than the cause. Usually people with APT are stuck in a state of chronic hip flexion, and would be well-served by doing more loaded hip extension (and stretching the hip flexors, of course). Not sure why you think more knee flexion work is the answer.