The biggest reason for that is that Apple, Qualcomm, and Samsung have designed their own custom cores, rather than using ARM's license-able ones. So most of the phones you see, instead of using Cortex A72, use Snapdragon 820 or Exynos M1. (Huawei's Kirin 950 uses A72 cores, though.)
One of the more interesting bits about the A73 announcement is that they're offering a new licensing level for SoC makers to customize their cores starting from an A73 base, and that Qualcomm has taken that license; so the next-gen Qualcomm SoC won't have the A73 name, but may have a lot of its tech.
One of the more interesting bits about the A73 announcement is that they're offering a new licensing level for SoC makers to customize their cores starting from an A73 base, and that Qualcomm has taken that license; so the next-gen Qualcomm SoC won't have the A73 name, but may have a lot of its tech.