Why doesn't bigcache consider designing expiration time for each cache key?

During the process of using and reading the source code, it was found that in each instance of bigcache, all key-value pairs can only be set with a unified expiration time. A `time.Ticker` is used to periodically check if the cache has expired based on the interval set by `CleanWindow`. So why wasn't a method considered to allow setting expiration time for each cache key?