The length of the engagement is not the one indicator of how the relationship will turn out.
There are lots of reasons to why a wedding date might not be chosen yet.
-Monetary. Weddings are expensive, even more so if the couple is young and doesn't have savings to draw from or wealthy families to contribute. They could be doing the smart thing by saving for their wedding rather than going into debt because of one day.
-Venue availability. If the couple has their heart set on a particular venue, they may be waiting for an opening. If they want a destination wedding they could be waiting for the right time of year to start planning. If they got engaged during the time they wanted to have the wedding it's at least 2 year wait to get a date there.
-Family/friend situations. If there is a sick family member or close friend, the couple might want to wait until that person is well again before subjecting them to the stress of planning a wedding.
-School/work situations. If the couple is in school they may prefer to wait until they have graduated before losing valuable study time (and even rarer couple time) to wedding plans. If they have just started new jobs, they may not be entitled to vacation time, thus no honeymoon, thus a delay until they can coordinate vacation time(s).
-Religious. If the couple is deeply religious they may be doing pre-marital counseling that is required by the church they belong to. If one of them has to convert in order to be married they may prefer to get that done with before making wedding plans.
I could go on but you get the gist...