There are a number of big reasons for this.
One is that a lot of anime is based on manga. So if the manga is still in the middle of being written, they can't just dump another season out. Used to be that shows would create filler episodes to wait for the manga to get far enough ahead for them to work with. The fans turned on that pretty hard though when shows like Naruto and Bleach had full or almost full complete seasons of nothing but filler. It's much easier, after all, to turn an already completed manga arc into a season or two of an anime than it is to pump out a brand new arc from scratch in manga form.
Another thing that affects anime seasons is popularity and sales. Now when I say popularity I don't mean viewership. From my understanding, in Japan, TV ratings for anime aren't nearly as important as ratings for TV shows are here in the states. Instead it comes down a lot to how much merchandise a series can sell. So if a show has good ratings but can't push a lot of merchandise or doesn't make good DVD sales, it's less likely to have a steady, reliable, release schedule for seasons.
Now that obviously doesn't explain everything. Like Castlevania, that feels like a case of them throwing out four episodes to test the water. That could have been a case of Konami demanding a trial to see if it could be popular or successful before wanting to commit to anything long term (especially considering video game adaptations usually are pretty bad.) But I don't know for sure.
But for something like One Punch Man, they didn't even know if the show was going to be successful enough for more than one season. Which is why it ends the way it does, so as to not leave a giant cliffhanger in case it wasn't popular and didn't warrant a second season. So from a business perspective deals we're only made for a season one, which also happens a lot in anime these days. There's just so much anime and no one knows what's going to be a big hit and what won't and no one wants to sign on for multi-season deals for a show that could be a dud, or leave a show on a huge cliffhanger that never gets resolved.
From my understanding those are a few of the reasons, anyway.
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