anyone can time travel, any old mage can bend time. the bronze and now the time keepers maintain order of the timeline.
the answer to "why hasnt the legion just attacked us in ancient times"
is the exact same answer to "why doesnt sargeras just cut azeroth in half"
they DONT KNOW WHERE AZEROTH IS and even if they did it would still take a very very very long time for them to get to us through outer space.
just like in real life outer space is huge and it takes a long time to go anywhere.
which is why had we simply shut down gul'dans portal in broken shore the legion wouldve been stopped
if azeroth knew where we were/was on his way no amount of anything would save us. he'd simply pick his sword up and cut us in half the end.