Because the question isn't about what a theoretical best system consists of. No one (including Musk) denies more sensors could theoretically be beneficial. Adding a gravitational wave detector to your car would ensure the car doesn't accidentally drive into black holes, making it more safe. This additional safety would not be economical.

Same principle with LIDAR. Adding $5k hardware per car and a ton of R&D doesn't seem economical to Tesla, since they think they can get pretty much the same with just their current sensor suit. Time will tell if they are right and what AP + sensor suite customers will choose.