It may be because there are more sealed collectors in other toy collecting fandoms. We don't have that many sealed TF collectors, at least not compared to other toy fandoms. And I'm talking about people collecting current toys to keep sealed, not people retroactively buying second hand unopened toys; because it would be sealed collectors of current toys who are complaining about Hasbro's recent decision to go with enclosed boxes.
It's relatively harder to find sealed Transformers, especially G1 TFs, compared to say finding sealed vintage Star Wars, G.I. Joe, Masters of the Universe etc. When I visited Lobos Collectibles in Melbourne on New Year's Eve, I saw walls lined up with sealed vintage MOTU figures, and only a handful of sealed TFs (a few of which were counterfeits and KOs). This is likely due to the fact that most Transformers collectors want to open our toys and play with them. With other toys like Marvel Legends, what you see is pretty much what you get; they all follow the same basic core body design. But with Transformers, we want to experience how these different toys transform. It's not good enough to just look at images of the robot and alt modes, the transformation in itself is a key part of the experience of owning that toy. And as such, we don't get too many sealed collectors in our fandom compared to others.
How many of us were collecting sealed toys in the 20th Century? I didn't start collecting spares of toys to keep sealed until 1997; before then I always opened every toy that I got. But in the 90s I already met collectors of other toylines like Star Wars and DC etc. who never opened their toys. Because every vintage Star Wars action figure more or less does the same thing; they have the same body plan with the same 5 points of articulation. Now that's not to say that it's not fun to open and play with these toys; I think the majority of toy collectors regardless of franchise opens their toys. But what I'm saying is that you're more likely to find more sealed collectors among non-TF franchises.
Disclaimer: Yes, I am very very much generalising here. I know that there are plenty of TF collectors who purposefully collect TF toys for the express purpose of keeping them sealed. What I am saying is that I think that sealed TF collectors are a smaller minority than sealed collectors of other action figure lines. And again, how many of them started collecting TFs sealed in the 80s? I knew a guy when I was a uni student in the 90s who already had a room full of sealed Star Wars toys which he bought in the late 70s, 80s & 90s (we watched The Phantom Menace together at the midnight premiere in 1999). He was the very first person I'd ever met who was a sealed collector, and also the first person I'd ever met who had a dedicated collection room. I still keep in touch with him today.
But yeah, the number of people that I know who started collecting toys sealed in the 20th Century I can count on one finger.
I have yet to meet a single Transformers collector who started collecting TFs sealed during G1 (1984-93).