They may have been mentioned/featured in things that last longer than the bows. Statues, pottery, writing on stone tablets.

Also I think they find skeletons with flint arrowheads in them sometimes too.

So it can be pieced together from lots of small, even partial data points.

But we find new stuff all the time that changes what we thought.

They found a 4000 year old bagless vacuum cleaner the other day.

That last one was a joke.