Software Install Hell - Why make me Reboot?

Rant of the Day - why the hell do so many applications require a reboot of your PC??? After having to reboot my new PC a dozen or so times last night just trying to uninstall 2 programs and install 2 new programs, I got pretty damn annoyed. As a home user, this was just an annoyance - for businesses with a large network, this could be a serious productivity impact issue.

So, why are these reboots necessary?

Googling revealed some Windows reasons, but apart from the "overwrites file in use" one, most seem to fall back to the installed program asking for the reboot. So why do the programs themselves think a reboot is needed? I can think of a number of alleged reasons:

1. The install overwrote a file in use, so it is Windows that wants to reboot
2. The reboot isn't really required, but the developers are just being paranoid
3. The developers didn't spend the time to remove the need for the reboot
4. Fundamental issues at the operating system level. Given the number of programs that don't need to reboot (some doing complex OS level integrations), I doubt this is the case

Reboots after uninstalling is an even bigger mystery to me - unless Windows is keeping a log of old file versions and trying to restore the previous version, the "overwrote file" shouldn't be an issue now. But if more than one update has occurred to the file in question, then this is just fraught with danger - which version do you revert to? I'd think the answer here is "don't do it!"

I think you should be able to just install some software, and it just works. No reboots. Even if it's necessary, be polite - don't force me to reboot.

From my experience, it seems to be commercial software that is more likely to exhibit this nasty behaviour - Open Source or software by 'enthusiastic individuals' seem less prone to require reboots. Is that because the former is more complex, or that the latter communities care more (or maybe get frustrated enough to spend the time to avoid the reboots).

Maybe in the commercial software development world, the issue of reboots fall into the low care factor category, as Company X assumes that their software gets installed once and that's it. Well, the reality is when I get a new PC and start installing dozens of my preferred programs, I get mighty annoyed having to reboot for every program thats so self-important that it thinks it's the sole reason I have a computer. Well, I'd like to point out that firewalls and anti-virus do NOT qualify for this - in fact you are a necessary evil and I'd happily dump in a nanosecond if it wasn't for all the cretins out on the net wanting to hack and wreck my PC.

Since I haven't seen much of a discussion about this previously, I'd be interested to learn more about the main reasons reboots are considered to be required, and how common this problem is on different operating systems - Windows, Mac OS, Linux, etc.