Because the timestamp and offset (i.e., a numeric indication of the time zone, as a difference from UTC)
aren't actually enough information to calculate the local time, pedantically.
Why not?
1. Leap seconds. They exist at irregularly spaced intervals based on the Earth's actual rotation, which is a) not perfectly consistent and b) ever so slightly, continuously, on average, slowing down. The offset would only allow you to figure this out, in principle, if you already had the UTC time - which has to be calculated from the timestamp in a complex way that accounts for leap second information retrieved from a database.
2. Time zones change over time. Some parts of the world use daylight savings time and others don't. It's possible in principle for a daylight savings time transition (which occurs on different dates in different parts of the world) to occur between two displays of the date, invalidating the offset.
3. Time zones change over (longer periods of) time. They've historically been created, removed or had their borders changed. Sir Sandford Fleming's system is now 148 years old (and there were others before it), and geopolitics happens.
4. Calendars. Maybe you don't care about, say, the eras of the Japanese calendar (https://en.wikipedia.org/wiki/Japanese_era_name), but some applications will require knowing about it. If you need to deal with history, then the conversion between Gregorian and Julian calendars will also depend on locale, because the Gregorian calendar was adopted at different points in different parts of the world.