"Time travel" is not the same thing as "going back in time".
All the races visiting Earth have the ability to time travel.
Time travel is simply the technology to use other dimensions to travel in.
Other dimensions have different values of time, so utilizing those of a greater value of time is the equivalent result to running up an escalator or throwing a ball inside of a moving car.
Sharing of Technology
Not all races are equally evolved or advanced.
Not all are given or permitted use of technologies ahead of themselves.
Not all races possess the technology to "go back in time".
Only some of the most advanced races do, and the technology is not available to just anybody of such a race for just any thing.
It certainly could complicate things if somebody "went back in time" and changed the course of some event.
This, and many other lesser and greater reasons, are why it's so important for every race to evolve in a socially responsible and trustworthy manner.
If a race is not responsible and trustworthy, then it is not allowed to advance, travel or share in the benefits of the union of advanced races.
A race who is as hostile, selfish and destructive as ours may never get past its own front yard.
In fact, it likely would never even survive itself.
However, many advanced races have had some similar difficulties as we have now in their own early beginnings and have overcome them - some with the assistance of their advanced neighboring races.