Why we can't install the ubuntu packages of older versions on new one?

We have learned working on computers in Windows era. So we are habitual of taking the backup of software packages. But this is not suitable for linux systems. In India, we are using windows XP since 2001, the kernel, library hasn't changed since then. So you can install a software on windows XP today which you have tried it 10 years back. But these software may not work on windows 7. The reason is simple, the environment has changed. The kernel, libraries and other supported software have been changed. The same thing happened with Linux based system. For e.g. for every 6 months we get a new version of the Ubuntu Desktop with new kernel and new version of some of the libraries. So the older packages wont work on it. It's best to install the packages using synaptic package manager. If the internet is not available easily then manage to download the Ubuntu DVD which has all the packages in it which are officially supported by Ubuntu. Once you have the DVD, then you need not to download the packages from the internet every time you install it on a new system. In the Software Sources check the option "CDROM with Ubuntu .....". Try this and let me know the results.