There are a couple of reasons why cannons were not widely seen, and they boil down to a few reasons: expense, logistics, and military doctrine.

Firstly, and probably most importantly, a cannon is expensive to build. The amount of material and effort that goes into a cannon could be better spent on creating matchlocks for the ashigaru, who, by the 'introduction' of gunpowder and guns to Japan (I say this in the loose sense of introduction by the Portuguese, rather than universally with contact with the Chinese previously), were numerous and composed the bulk (both percentage-wise and by sheer numbers) of Sengoku-era armies. I mention this in the wider scope of what siege warfare looks like in this short post, which helps transition to the second point.

Cannons would, in theory, be mostly used during a siege to break down a castle's outer walls, or just make it a really terrible day to be outside for the defenders. But, looking at how castles are laid out, it becomes almost trivial to see how this would, in practice, not be such a good idea. The amount of artillery and raw material (which I'll touch upon in a bit) you would need to dedicate into blasting apart any of those castles into rubble would be so much so that, again, you'd probably be better off equipping your soldiers with matchlocks that would work just as well.

Furthermore, it is likely that, due to the quality of the iron available to the Japanese, it simply would not be feasible to manufacture cannons on a large scale, never mention the round shot/projectiles to load into said cannon. Considering as well that for each round shot you need at least a quarter of its mass in gunpowder, that's even more resources spent on something that would again be largely ineffective in most cases.

That being said, you touch upon a good point that cannons were 'present' in the region - this is not so much due to Sengoku armies fielding them nor manufacturing them on any wide scale, but (in the case of European-like cannons) were most likely found from wrecked ships or introduced by the Portuguese. Yes, the Japanese themselves dabbled in 'cannons', but nothing of the sort that we are used to in Europe.

/u/Suedie