J and YA titles tend to be a little more flexible with the covers (so does nonfiction), but even those are centered the vast majority of the time, from what I see. And if it's not, they'll do a title that's not fully centered, but still uses centering somehow, and does clever stuff with the composition of the image to still make where the title is *feel* like the center, so that the image is still shaped around the title, rather than the reverse.

And most people who are still figuring out kerning can't do that stuff.

(Although also, I think most of the YA I deal with is fantasy/SF/historical/etc. and that tends to be more in line with the styles used for adult fantasy/SF than the 'realistic' YA is, which often wants to look like 'literature', which means taking more risk with the design.)