Why not all Applications are Portable and require Installation?

We need more and different kinds of programs and applications in our systems. Installing these applications also helps us in our several situations. Whenever I come across a new program, I first check its size and the space taken after installation.

Today we also have the option of going for the no-installation programs called as Portable Apps. These portable applications are so easy to use and carry that we just need to take them with us and use them anywhere on any computer. Then the question that came to my mind is If using the portable apps is possible and easy, then why not every application is portable and why do they require installation?

With Windows came the concept of config.ini files which are nothing just the configuration files of the tools having information about the program. But managing these files was somewhat difficult, thus came Registry which replaced the config files. Now the configuration settings of the programs were stored at the same place called as Registry. But not all programs removed the registry when they were removed from the system.

But I guess the idea is to keep the entire configuration files of the tools at the same place, their other dependent files at other and the user data at some other location. Also creating a portable app of a tool has its own disadvantage. If an application is huge like the MS Office or the iTunes, then running the portable version of the app will slow down the system and also result in slow startup. Moreover the portable app with all its config files at a single place (called the portable app) will consume lot of space.

Thus to get instant startups we have some programs that still require those horrible installation procedures. What do you think?