I think we have to take into the equation their time and place (like in most other regards of this movie). It's crucial.
What would have been the alternative for them? It's easy (well, at least easier) to quit an unfullfilling/unhappy relationship when you have at least theoretical alternatives. And even then it's hard, see Arad's story.
But for Ennis and Jack? What else could have been for them, if not each other? For Ennis, I'm sure there never could have been any alternative to Jack. The alternative for Ennis was what he was left with at the end of the movie: nothing but being all alone and desperate.
And for Jack?
Randall? What do we know about him? Next to nothing. Had he been willing to share a life with Jack? We don't have any clues, except that Jack spoke about a neighbour fella to his father. But cruel as his father was, he was not completely wrong: Jack had spoken of bringing Ennis to LF for years and years - but nothing happened. Mexican prostitutes? Not halfways an alternative to what he shared with Ennis. Hoping to meet someone else? Well, how secure was Jack in his sexuality, what inhibitions and shortcomings did he have? We tend to concentrate on Ennis's shortcomings, demons, fears. But Jack had also his issues.
Then think of the mere chance to meet other gay men in their time and place. I don't think LF or Childress had a gay bar at that time. Maybe go to Denver, like he supposed in the story. But would he have divorced Lureen, separated at least partly from Bobby, left the convenient life in Childress only for the vague possibility to meet someone else some time in Denver or any other bigger town? I think not.
He didn't do that for the chance to live more near to Ennis, for the chance to meet him hopefully more often, for the hope Ennis might come around eventually (btw, no accusation here, just stating the facts). Therefore I'm sure he wouldn't have done it (leaving his live in Childress) for less than that.
Would they have been able to give their lives a new direction, to start fresh with a good chance to find happiness without each other? Ennis clearly not and I think the same is true for Jack. The main reason I haven't even mentioned yet: because their love for each other was true and deep and mutual.
So having only a bit of the one you love, having only a few weeks of happiness a year was better than to have nothing.