In some cases, I find some question titles followed by [Duplicate], I am trying to know if Stack Exchange merges the duplicate questions later or not? If yes, why? If no why?
There is no automated system that merge duplicates, but moderators can merge questions manually. This is rare, but still in use.
I don't think this should become automatic, duplicates are good as they are, and sometimes there are subtle differences so different set of answers is good.
If you do see a question with good answers which is closed as duplicate of other question also with good answers, you can just flag the question and ask a moderator to merge.
Merging is messy.
In a few cases it's handy, but having duplicates leading back to a canonical question leaves a set of breadcrumbs to the canonical question. In some cases it's actually possible to distinguish a 'dupe' from the original enough for it to be a standalone question.
As a moderator, practically speaking unless I somehow find 2 posts with the same question, and utterly fantastic yet distinct answers, I won't even consider merging. I think I've done less than 2-3 merges in my time as a mod.
It's also really hard to undo a merge. Having oversight from a human means at least we have people thinking hard - both from the perspective of "whether this is actually a dupe" and "whether merging is worth it". Its often worth a hard think and discussions before doing it.
As such? Its a useful tool in very rare cases, but for most part dupes pointing at a master question works well enough, so we rarely merge.