Since the game has been announced, Bethesda kept showing outdated builds in their trailers, and then come out and say "The new build looks a lot better!"

Well, if it looks a lot better, why not showcase it?

All the gameplay videos in the last month have been from the "E3 Build"... Which is probably from May or even before.

I just don't understand this. Surely you want your game to appear in good light?

It's fairly common in the game industry actually. The last thing a game developer want to do is to show when the game bug out and show glitches or even crash, so an older build they know is fairly stable is used instead of the new build that might be unstable.

As for trailers, it's possible that the raw trailer footage was taken months before the trailer is finalized and released to the public.