Better Call Saul is available on ~15 different services including Netflix, Amazon Prime Video, YouTube, and AppleTV+. It would never have occurred to me that I should watch it on a separate AMC service.
I don’t remember when, but at some point I tried to watch the then-most recent season and it didn’t seem to be available anywhere legally.
Now, there are some places you can watch it, but AMC isn’t one of them.
I also checked another flagship AMC show, The Walking Dead. Not as critically acclaimed, but huge ratings. You can watch only season 11 on the AMC+ service.
Netflix has the old seasons of The Walking Dead and Better Call Saul. I’d hope AMC is getting paid handsomely by Netflix for the rights, and that may be the right move financially. But selling the crown jewels seems incompatible with a successful AMC+. I recall reading that Netflix found that the first two seasons of a good show help subscriber numbers, then the benefit of additional seasons drops off (and costs increase) such that 3+ seasons only pencil out for the most popular shows. From that perspective, the AMC+ offering of limited runs of recent episodes from their most binge-worthy shows is a doomed strategy.
It would not have occurred to me to watch Better Call Saul via a middleman when the technology to obviate them has been widespread for more than a decade.
The last season isn't on Netflix or any of the other services you listed though. Personally, I suspect that's because that last season was GoT season 8-tier terrible and they're trying to get as many new fans to watch the older seasons before the masses recognize that the show is dead.