But they don't have a time machine any more. It was destroyed when Thanos' ship flew threw it, and then Thanos nuked the building it was in.
And as they told us in the beginning of the movie, Tony Stark is the only brain on the planet capable of solving the time travel problem and building a working time machine. And now he's dead.
"But wait a minute...wouldn't Tony have left, you know, blueprints or a flash drive with an instruction manual for building the time machine on it?"
Yeah, probably -- but again, any copies of the plans for building the time machine that were kept at the Avengers' compound (whether paper files, stored on computer drives, or otherwise) also would've been destroyed when Thanos obliterated it.
So the only disbelief we really have to suspend is the notion that Tony wouldn't have backed up the plans somewhere offsite, which I guess strains credulity a little. But then again, it's a film series about superheros battling a genocidal alien, so...