I don't think it's far fetched to think of it as bleeding-edge technology that only a few (of a certain species) had control over at the time. It would be similar to us meeting humanoid aliens and watching them master nuclear fusion.
It takes some effort and patience (mostly in preparing a good environment for a spark to light something small and dry on fire), and you need to know what are good conditions for a fire to start (which may not be obvious if you've never had fire), and you need a desire to make a fire as opposed to fearing it; so there's some barrier to overcome in inventing/attempting the process, but once that's done and you see how others are using and maintaining fire (even if the 'magic' initial process is obscured) then the technique would spread out like literal fire.
It takes practice but steel & flint isn't necessary, here's a video of a friend of mine who does what you're saying, teach kids survival skills. She uses particular kinds of wood you would need to learn to recognize but of course people have been doing it for a million years
Warning: very enthusiastic audience; having been off the grid for a couple weeks, seeing a fire started by hand was big entertainment
Then if you do manage to get a fire, you have to have enough resources around to fuel it and get some coals for your giant sloth sirloin. If you camp in an area over time, eventually you will exhaust all the available firewood. Plus, every time you light a fire you risk other groups of people and predators poking around for that giant sloth sirloin of yours.
If I've been getting by ok eating sloth sushi fresh off the kill for millions of years, why even risk it? Homo sapiens probably didn't have the gut microbiota to manage. Food poisoning could be a death sentence back then.
In terms of fire making, like all other survival skills it's about prep. I have pockets full of dry tinder, straw and splinters. They would have too, and caches of dry wood.
One explaination here might simply be that these sites became untenable in the winter and were used as summer hunting camps. Short summer night fires would leave a fraction of the evidence of a constantly stoked winter hearth.
Yes, it takes time and effort to collect the right fire starting material in wet conditions - you might need a couple hours and a longer walk, but that's what hunter-gatherers do much of their day anyway.
And yes, it takes some "wilderness lore" specific to your local environment to recognize what's suitable and what's not, what's available and in what locations. That can be transferred reasonably quickly (i.e. the very basics in an hour or so) by showing stuff on-site but not easily over a ten minute youtube video that's likely taken far away in a different environment; but again, getting to know the local flora and fauna is a big part of young hunter-gatherer 'education' no matter if they use fire or not.
That said, I agree with OP(s) post, in general. A fire isn't really that difficult to make with the right tools (a flint would be great) "Friction fires" are indeed much more difficult, but if it's important to you, it's relatively easy to learn.
Also, once you get a good fire going, there are ways to mitigate these issues. Coals can be re-used, and wood can be dried. This is basic wilderness survival. I wouldn't necessarily count on an encampment running out of wood. Some areas have (or at least had) plenty.
So the question of whether a fire is easy to make isn't really a yes/no thing. It depends on your access to materials and level of experience.
People saying its easy isn't taking into consideration that this is knowledge that took a really long time to figure out and effectively pass on. Knowing that rubbing two of the right sticks together for long enough will produce friction that can lead to enough heat for a fire isn't something that a person with no knowledge easily comes up with by themselves.
I'm not trying to nitpick here, it's just something that's worth pointing out. This was news to me when I found out, and relevant information if you were the end up in a "state of nature"
And just like nuclear fusion, if you really fucking need it, and you dedicate a lot of effort to it, it can be done.