Because, there are tons of programs that you won’t ever need or use.
There are levels of “importance” in a distribution. The most important core packages are the ones that are absolutely necessary to keep the system running. These are maintained by Canonical (Ubuntu) and receive the guaranteed 5 years of support.
The “vast majority of software” is the “optional” software from a variety of maintainers (Ubuntu community). How much these packages are supported is, to my knowledge, entirely up to the developers. Though, this also means, it does not relate to LTS in the first place, because it’s not up to Canonical (Ubuntu) how much or long they are supported. At least, I am not aware of a contract that developers have to sign regarding minimum support terms.
If “universe” is for all developers not directly affiliated with Canonical (Ubuntu), then the things I said in the previous paragraph most likely apply here. It’s all up to the vast majority of individual developers, how much their software is supported and for how long. It’s not related to LTS, at all.
That’s what I am talking about. The core packages (the most important ones) are released and maintained by Canonical (Ubuntu). These are the ones getting LTS. However, third party software from third party maintainers do not abide these rules. It’s a bit like on Github. Either the software is maintained or not. They do not sign a contract stating, that they need to provide LTS, in any way, as far as I know.
No, I think this is a purely theoretical assessment and does not apply to reality. For example, there are tons of reasons why you would still apply an upgrade or fresh install more frequently. For example, you need the newest version of a software, but the dependencies the old Ubuntu operating system provides, are too old.
Or, something happens and you have to do a fresh install anyway, because you need that one thing and restoring a backup does not fix it, since you still need that one modificaiton.
It’s also usually better and more secure to upgrade more frequently.
Still not sure, why you are forced to do a fresh install, instead of doing proper upgrades, however if that is such an issue for you, I would instead recommend that you automate the fresh installation so much, that it would take just a bit of effort to do a fresh install. This way, you can do it very frequently, but with the pain minimised.
There are also projects available, that are working on doing something like this pretty easily.
Again, I think this is not the right question. It’s really not that important. It’s better either way to just upgrade more frequently.
That said, it is real 5 year support, but only for packages Canonical (Ubuntu) provides and maintains. So, it is real. It’s just that the core packages are just a small subset of the total packages available.