New phones are also not tied to the x86 platform, and generally run software that has been optimized from the start for performance.
There are other considerations too. Phone usage patterns are way different to computer usage patterns. The vast majority of phone usage occurs in small blips of activity during which execution can be optimized for the CPU to race to sleep state as soon as possible. Computers don't have the same usage patterns at all, and have a legacy of first and third party software (on all major platforms) that have not yet been optimized for performance.