Most of the time when constructing a FDateTime, you’ll be setting it to FDateTime::Now() so it is slightly faster without setting the Ticks to 0 first.
Do a search in the UE source code and you’ll see what I mean…
There is also another constructor which takes a “Ticks” argument if you do want to initialize.
There are many reasons to not initialize properties in a default constructor, usually for performance reasons. In the case of FDateTime it probably makes little sense, but for something like FVector which is used often, it makes more sense.