While it may be possible to copy some of programs in this manner, it is not the case for most software packages you may have installed on your old system. Re-installation of software packages from the original install media ensures that everything the program needs to run is where it needs to be. Settings, dependency files, etc. all need to be available for the programs to run and all of that is put in place during installation. Simply copying the directory where the program was installed to on your old system to your new system will not do this.
It is also important to make sure that the software you are installing is compatible with the operation system on your new computer. If you are upgrading your operating system as well as your computer there may be newer versions of software that need to be installed.
In short, it is not recommended to copy programs in this manner because they are not guaranteed to work.