If I had to guess, I would say that 90% of AK-47s that you see are really AKMs. The major thing that the AKM did to improve over the AK-47 was have a stamped receiver as opposed to the milled receiver, which made the AKM significantly easier to produce over the AK-47 (because milling receivers takes a long time).
The AKM replaced the AK-47 in the 1950s. It is all around better. Here is a blog that lists all of the improvements that the AKM has over the AK-47. It is pretty accurate as far as I can tell.
What made people call an AKM an AK-47? Is it because they look the same? Even so, why did the AK-47 rise to popularity even though the AKM is the more popular AK rifle?
Is it because they look the same
Yes. The difference is pedantic and meaningless to 99 percent of people, who don't know, or care, about the differences between the original version and the Modernizirovanniy that came out a decade later. Far more far-flung firearms from the Kalashnikov family, such as the AK-74, also will get lumped together as "AK-47s" in the popular mindset because, again, few people really know or care about the distinction. In his history of the gun, appropriately titled "The Gun", C.J. Chivers lists over a dozen different arms which are often all called 'AK-47s', beyond just the AKM to include Type 56s, AMDs, M70s, and so on, and that doesn't even get into something like the RPK, which in the end is just a "heavyweight AK-47" if you want to annoy people.
These are guns that owe their genesis, in some part, to the original, but are better thought of as the Kalashnikov family than a single gun of course, but as Chivers notes, "the name has become, in public discourse, a shorthand for the entire family of firearms". And of course, I'd note that even if 'AKM' for whatever reason was what had caught the popular imagination instead of 'AK-47', we'd merely be moving the goalposts there would still be plenty of room for complaints about "Why does everyone call the MPi-KM an AKM!?" After all, the early American experience against the AK-47 was neither AK-47s nor AKMs for the most part, but mostly Chinese Type 56s that ended up with the Vietnamese, but it was all 'AK-47s' to them.
It is the same reason that if you show someone this almost no one is going to exclaim "Hey, nice C7A2!" They will just assume it is an M4, or an M16, and its doubtful most people could tell the difference between the latter two at a glance either. Because again, most people don't know, and don't care, about the difference.
So there is no single 'event' to peg it too, it is simply a matter that the AK-47 was first, and outside of the small circles where people are really interested, it is more than sufficient to just use 'AK-47' as a catch-all for variants, and to be perfectly frank, not particularly objectionable either.
