Performance. get_current_cpu_cache_line_size would need to run some code to determine the cache line size, and that code takes longer to run than using a cached value.

Along similar lines, if you have an optimized routine using specific CPU instructions, you don't want to call CPUID (or equivalent) on every call to find out if you have those instructions; you want to call it once and cache the answer. If it can return different answers on different CPUs in the same system, you need to use a different mechanism instead, such as asking the OS for the least common denominator features of available CPUs, or notifying the OS that you need to run on one of the more capable CPUs that has the feature you need.