People talk about adding new scenes to the '86 movie like it's something that can just be done on a whim without any issue, akin to live-action films restoring deleted footage that was filmed at the time of the original filming work but went unused in the final cut, or like Disney adding new scenes to their animated movies. But what people don't realize that is that such comparisons are like apples and chainsaws.
The reason live-action films can restore deleted footage to later releases of movies is because the footage already exists. All that the DVD/Blu-Ray companies have to do is acquire said preexisting footage, clean it up, and edit it back into the final cut of each movie as seamlessly as possible.
The reason Disney can new scenes to their animated movies (think like the DVD release of The Lion King adding in the "Morning Report" song sequence or the DVD release of Beauty and the Beast adding in the "Human Again" song sequence) is because Disney is an all-in-one media production company unto itself, with its own animation, music, and voice production studios all under its belt.
The 1986 Transformers movie, however, was produced by two dead production companies who didn't keep all of their production assets (as evidenced by how fans have had to track down all the behind-the-scenes materials themselves over the decades), and who didn't animate any extra scenes that were cut from the movie besides one extended camera panning shot that follows after Astrotrain's departure from Autobot City. The animation itself was outsourced to Toei, not done in-house like Disney's animated films are.
So, in order for there to be new scenes added to the movie, Hasbro would have to reach out to some production company and pay them to recreate the original animation assets of the movie from the ground up, commission the creation of new scenes using those assets, and, if Toei were to be involved for true authenticity, outsource the actual animation work to them. And all of this would be on top of needing the first production company to take those new scenes (which would likely be created digitally in this day and age in contrast how the rest of the movie was made) and edit them back into the movie as seamlessly as possible, which would also require an editing of the score and soundtrack to accommodate the new scenes slotted into the picture. This in turn would also likely require new music compositions, be they completely new tracks to modified versions of the original score. For true authenticity, this would require reaching to Vince DiCola and commissioning new music work from him.
The more and more all this goes on, the more and more money all this is going to cost. At that point, with all these extra expenses, it begs the question from an executive corporate perspective if going to such expenses for a few measly minutes of more runtime is really all that worth it for a movie that they know they can already make plenty of money on as is, without adding any new footage, since TF fans will just eat up the newest release of the movie like the cattle we all are.