They do it because it's fun for them and they have the free time to do it. They'll swear up and down it's because they have to have absolute control over every aspect of their system because that's so, so, so important and necessary to everything they do, but in reality it's because they've got the time to do it and they're fundamentally doing it because it's fun and interesting for them.

Nothing wrong with that, that was me at one time. Of course, at the time I too swore up and down that all the time I spent maintaining my system(s) was because it was somehow really important for me to do so, and I only had a vague suspicion that it was really just me having fun learning about stuff and hacking around on all sorts of low-level things that I wouldn't get to mess around with otherwise. I still do it when I find the time but it's usually with a little more self-awareness about my motivations for doing so. :)

Is it necessary? Of course not.

However -- to some, me included, maintaining an operating system environment is somewhat of a hobby. Operating systems like Gentoo provide standardized ways to configure how an application is built, and more importantly it provides ways to configure what compile-time features are (and are not) included in the binary. While most people wouldn't find the effort worth the yield, there are people who spend years of their life meticulously making a ship in a bottle. People enjoy it anyway.

Another draw to Gentoo for me is the way they handle package management. You can stick to stable, and packages will be relatively well-tested and mature, or you can choose unstable which will offer you bleeding edge packages. You can also have a stable system, and mark only a few packages unstable. Not many distributions offer a system like that. They are usually completely stable, or don't offer a way to keep most of your system stable while letting a few packages be unstable.

I've used Gentoo for a long time, and my maintenance is nearly nothing these days. Few problems arise (a new GCC version breaks a program or something), but I've actually gained enough practical knowledge to go about fixing these problems rather painlessly for the average case.

Can't recall where I saw it, but: Novice users use the stock kernel because it just works; power users build their own kernel and optimise it to get every last marginal scrap of power out of it; advanced users use the stock kernel because it just works...

If you're somewhat familiar with Linux the time investment isn't really that bad. I've wasted more time over the years due to weird Windows quirks than I have configuring Gentoo.

This is my impression too. Every now and then I run emerge --sync && emerge -avDNu world, do a quick sanity check, then leave it alone.

I've spent enough time on other linux distros chasing down PPAs, or trying to diagnose weird issues that it refuses to log because it might intimidate the user.

I think it's like owning a classic car that you need to pamper all the time. It's a tool much like your computer but some people love tinkering.

Some people work in areas where fine-grained control of their environment (library versions, kernel options, etc.) is a requirement. Others do it to learn. IMHO you could benefit from being less hasty to judge and dismiss others.

While I'm not a kernel builder, I do aim for the barest possible distribution out of the box. I'm most happy when I have a basic install that includes vi and apt only.

That way I know exactly what dependencies I am dealing with and what all I need to be aware of when configuring any tool.

Might not be the same, because I am in the dist kernel upgrade group.

I could take public transportation and save a whole lot of money (gas, payments, insurance, etc.) but I find that having the flexibility to go wherever I want whenever I want is a good reason to keep my SUV.

In addition, I could save a helluva lot more money by not owning a motorcycle and driving the SUV. To me, though, it's worth every penny.

I think you're looking at a vocal minority. :)