I've started watching anime over the last year or so, and I've finished somewhere around 80 shows. I've noticed that more often than not I'll finish the available content and when I look up if it will be continued for more seasons there is no information either way, even though it's been years (often 5+) since the last season was released. If they don't plan on making any more of it, why don't they just say its going to be cancelled?
This is because, from the perspective of production, not planning to make more episodes is not the same as definitely will not make more episodes. In the same vein, not planning to drop your phone on concrete is not the same as definitely will not drop your phone on concrete. There is always a chance that an anime series can come back or make more episodes, in the same way that there is always a chance that you will drop your phone on concrete.
Anime production is quite expensive, often with narrow green-lighting windows to actually make content. So, the main hope is that the show that you're producing can gain and garner enough support to justify the expense that it's going to take when it's produced.
If it doesn't...then it makes logical sense that it won't have a continuation. No funds to continue production is a sure-fire way to have a one-and-done anime.
(This is why it's especially important for you to support the series you want to see more of by buying the DVDs/BluRays and the merchandise.)
I would argue, though, there have been very few - if any - animes that have actually been outright cancelled. Because of the narrow way that animes are produced to begin with - only enough budget for a single cour which may cover a portion of earlier work - it's always been, in my experience, either delays or hiatuses or something else besides an outright cancellation.
Furthermore, you never know what the future holds. Some voice actors who weren't available or wouldn't be available to take on those kinds of roles in the moment may free up at some point in the future, so there's always a possibility that the work (if there's more reference material to go off of) could be continued in the future.