And what specifically prevents you doing so? After they recently added multiple sound tracks and pan/zoom, for the vast bulk of users the only obvious omission is a timeline or some other way to vary individual slides' duration. Maybe 4K output should be there, maybe export to Premiere Pro too (same method as Clip?), but beyond those we're probably getting into individual wants - and you can never please everyone.
Well, that timeline oversight is a huge barrier. Without it ... how does 4k capability even factor in? A plain vanilla slideshow is even more so at higher resolution. Plus, if you consider, prior to 2006 very few photographers knew they wanted Lightroom either. Let alone what feature sets they could foresee in future development. Maybe a lot of users aren't aware of what they want in the Slideshow module because they consider it an exercise in futility. Maybe if it were more robust, it would become much more popular if it were no longer treated as a cursory attempt at a solution.
You fully understand what was possible in Aperture as far as slideshows are concerned. It was not an exorbitant or luxurious coding process to offer such features, yet the only limitation was on the individual user's imagination. I can't possibly see how similar functionality could be considered unnecessary individual wants. Though, we did discover that Adobe's interpretation of user metrics was considerably off the mark with the v6.2 fiasco ... it could be understandable that they really don't have an accurate grasp on what users really do desire.
Indeed Adobe added multiple music tracks ... but no method to craft them into a coherent flow to match the mood of the specific images for a particular show. No method to customize the duration or segments of the music the user would prefer to use. It's either all or nothing. That's not creative in the least.
Likewise they added pan and zoom ... but no method to customize what is the focal point content of the image at hand, if it isn't dead-center, oh, well ... it is a one-size-fits-all solution to match the you-can-use-any-transition-you-desire-as-long-as-it-is-disolve.
I'm sure it would be nice to have the ability to export to Premier Pro ... but I would guess the number of Lightroom users that also have PPCC installed is a much smaller group and could fall into that niche individual wants group - and as you say ... they can't please everyone.