If you look at the debuting positions of artists like Mariah and Whitney, I've found that they usually debut low on the Billboard 200 and then skyrocket in the following weeks. That being said, the album had already yielded successful singles at the time of their debut (examples are Mariah's debut album and I'm Your Baby Tonight by Whitney). I'm wondering if maybe the public wouldn't be aware of the album release during the first week, and then in the weeks to come ppl would see it on shelves and realize it was out?

Anyone have a reasoning for this (sorry if I didn't articulate all that well)?

No pre-order, no previews, people had to decide whether to pick an album or not based on the songs they played on the radio

I've noticed this as well. I suppose that it's because promotion was focused on longevity rather than big debuts. There were also less ways to find out about an artist's new album with social media not yet in the picture.

This is a secret but I will say it: THRILLER debut with like 30,000 copies in its first week... it was wayyyyyy different back in the day.

This makes sense...it also explains why filler was so prominent in albums during that era

The internet wasn't used to promote albums and create hype, and lead singles would come out like a month before the album release.