I want to preserve my /home directory but it is not on a separate partition. I can move it to a separate partition if it is really necessary. However, I not sure why an OS installation can't just keep the current /home. Is it possible to reinstall the OS and keep the current /home on the same partition that will contain the OS?

Thanks,

It might be possible, but you'll also keep everything else that isn't same-named as new files that come in.
I wouldn't be game to try it.

Separate partition is the way to go. Works fine with the distros I've tried, but you still need to be aware of potential changes in packages you need to re-install.

problem with this thought being is to ensure all the distribution specific files are wiped clean, one should do a FORMAT of the drive on which the operating system is being replaced. formatting does not discriminate between OS files and user files, this would be impossible, which is why it is recommended to keep /home on a separate partition, well that and it becomes easier migrate the /home partition to a separate drive if necessary, or migrate /home to a larger drive without re-installing the os, or inversely re-install the OS without losing user files.

Some distros will do an update which would leave /home untouched, but not many offer that facility; I can't remember about Ubuntu. Remember that Linux is not Windows. The distro has all those programs sharing all those libraries. Most distros install 2000 or more packages, most containing several files. Writing software that can update some files, delete the things that are no longer necessary, add new material, and make any necessary links is a vast job. Doing it, and guaranteeing that it will work (especially if the user has altered anything), is more than most development teams have the time or the courage for.