If this has a real-world/measurable/etc. impact why isn't this set by default? Are there potential side-effects? Is it set in some distros but not others?
In general, the timezone is set during OS setup, and the system is left in a state where it's up to the applications to figure out what to do. For example, you might configure Apache (yes, I am old, leave me alone) to use a particular timezone. But if Apache senses an env var it may choose to override the configured value with what's in the env var. Or SSH might be configured to pass along all env vars, including TZ, which in all honesty it probably won't even if you tried, but it could, and then the destination server's application has the wrong timezone.
Point is, it's safer not to mess with env vars unless you need to.