Why do people keep doing this? At this point I consider it a sign they'd rather be fancy than usable, which I can only imagine must be how they write their other software as well.
Is there a better alternative? Does something like a wordpress theme support proper scrolling out of the box? I am working on a blog and know next to nothing about this :)
You don't need "support" for scrolling. It is supported by the browser, out of the box. The website hijacks scrolling with JS by introducing "smooth" scroll, which is a big no from the UX perspective.
What they think is "smooth" then feels like "my finger is wet and the screen isn't properly reading swipes" on a phone.
Your readers will have picked a web-browser that supports scrolling the way they like it. The less you try to interfere with the way people choose to visit your webpage the better.
I know next to nothing about websites. Sorry if this is obvious, but what do you mean by that? I was able to scroll and read it fine on an iOS device with Safari. What is the alternative or best practice nowadays for scrolling websites?
The website is using a javascript library to override the default system scrolling behavior. It creates what feels like, at least to me and some others in these comments, an unresponsive scroll animation that does not match the behavior of essentially all other apps on my device. This is called scrolljacking, and is generally discouraged as it tends to ignore the behavior the user has configured at the device or OS level.