We care more about long term security updates than full long term support
We like running so-called 'LTS' (Long Term Support) releases of any OS that we use, and more broadly of any software that we care about, because using LTS releases allows us to keep running the same version for a fairly long time. This is generally due to pragmatics on two levels. First, testing and preparing a significant OS upgrade simply takes time and there's only so much time available. Second, upgrades generally represent some amount of increased risk over our existing environment. If our existing environment is working, why would we want to mess with that?
(Note that our general environment is somewhat unusual. There are plenty of places where you simply can't stick with kernels and software that is more than a bit old, for various reasons.)
But the general idea of 'LTS' is a big tent and it can cover many things (as I sort of talked about in an entry on what supporting a production OS means to me). As I've wound up mentioning in passing recently (eg here), the thing that we care about most is security updates. Sure, we'd like to get our bugs fixed too, but we consider this less crucial for at least two reasons.
First and most importantly, we can reasonably hope to not hit any important bugs once we've tested an OS release (or at least had it in production for an initial period), so if things run okay now they'll keep running decently into the future even if we do nothing to them. This is very much not true of security problems, for obvious reasons; to put it one way, attackers hit your security exposures for you and there's not necessarily much you can do to stop them short of updating. Running an OS without current security updates is getting close to being actively dangerous; running without the possibility of bug fixes is usually merely inconvenient at most.
(There can be data loss bugs that will shift the calculations here, but we can hope that they're far less common than security issues.)
Second, I have to admit that we're making a virtue of more or less necessity, because we generally can't actually get general updates and bug fixes in the first place. For one big and quite relevant example, Ubuntu appears to fix only unusually egregious bugs in their LTS releases. If you're affected by mere ordinary bugs and issues, you're stuck. This is one of the tradeoffs you get to make with Ubuntu LTS releases; you trade off a large package set for effectively only getting security updates (and it has been this way for a long time). More broadly, no LTS vendor promises to fix every bug that every user finds, only the sufficiently severe and widely experienced ones. So just because we run into a bug doesn't mean that it's going to get fixed; it may well not be significant enough to be worth the engineering effort and risk of an update on the vendor's part.
(There is also the issue that if we hit a high-impact bug, we can't wait for a fix to be developed upstream and slowly pushed down to us. If we have systems falling over, we need to solve our problems now, in whatever way that takes. Sometimes LTS support can come through with a prompt fix, but more often than not you're going to be waiting too long.)