I downvoted the question because it can only be answered with speculation. But what I would speculate is that your dislike for instrument-playing-vocal-melody has little to do with inherent problems of that, but instead:
Such performances are, more often than not, musically just quite poor – both poorly arranged and poorly played. They tend to fall in three categories:
- Amateur ensemble that normally plays classical music, big band jazz etc., attempting something “modern and hip” for a change.
- Production company trying to make some safe cash by recording famous songs that people searching for some unintrusive background music may be interested in.
- Virtuoso showing off their solo skills but trying to keep it digestible to their audience by selecting a well-known song.
There's nothing wrong with any of that, but there are problems, starting with the arrangement. In 1. and 2., this will be done by someone paid just to get the job done, likely not with much enthusiasm (and not necessarily without much artistic skill to begin with). In 3., the arrangement is done by the soloist whose real goal it's just to underpad the solo, without much concern for making it a balanced and consistent whole.
Then there's the performance itself.
In 1.: an amateur ensemble can obviously not be expected to have the level as professionals playing original material. In particular, amateur ensemble players don't tend to have a very creative-artistic approach to their music; they may do alright when just playing something like in a professional recording they know, but uninspired when it comes to bringing up something different.
In 2.: that would be done by session musicians, likely not paid very well and largely uninterested in the project. They'll see it as just a job, and play it just good enough so there's nothing particular to complain about.
In 3. we have kind of the opposite problem: the virtuoso will hopelessly overplay the part, with excessive ornamentation and other expressive whose purpose it's less to be musically tasteful than to show off technical prowess.
Of course that doesn't mean all such performances are bad. Here are some instrumental-covers that I like a lot:
Category 3. Jeff Beck is an incredible guitarist. His unique sound may not be everyone's taste, but he definitely uses it in a very controlled consistent manner. And after a career like he had, he certainly doesn't need to prove anything to anybody!
Not sure if this is 1. or 2.. I think most of the players are amateurs, and it's not perfectly executed, but it's definitely clear how much enthusiasm for this particular project is in the room.
3., so very much. It's obviously ridiculously overplayed, but that's the point.