system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do we need to install a program/videogame on our computer instead of just opening it.
gpt
It's a couple of things, and it's all in the name of efficiency.

First, to make programs take up less space on the CD or be easier to download, a lot of them are compressed.  This is good for space, but it's very slow to uncompress something, so they can't run like that.  The installer has to uncompress everything to get it ready for use.

Next, the program needs to tell Windows what it can do.  That way, when you install a new sound program, you can click on your MP3 music files, and Windows will know that the new program can play those songs, and will use it to do what you're asking.

Once that's taken care of, the program needs a way to store settings.  You can't really store settings inside a program file like an .EXE, because these files are very complicated and require special software to modify.  So when you install it, the program makes a special file to store all its settings and temporary information.  Since it's not an EXE and it's not actually *running* in the computer, it's very easy to make changes to.

On the same note, the program needs a way to update.  Since you can't change the EXE file, you can't really update it without replacing the whole thing.  If the program is broken down into a bunch of smaller files, an update can just replace one or two of these little files to make everything work better.  With a single file, the update would have to replace the whole thing, even the parts that hadn't changed.  That would take extra time and be harder to download, so programmers like to avoid it.

Finally, a lot of the really basic things a program does, like making a window or a dialog box, or certain types of calculations, all require special computer code.  But a lot of this code is the same for every program, so instead of making every program come up with its own method for doing these things, the simple tasks are made into what's called a "library."  This is sort of weird naming, because they're more like books *in* a library.  Each book tells the computer how to do one simple task.  So, when Microsoft Word wants to make a dialog box so you can set the font size, the Word programmers don't have to write their own font picker.  They just tell Word to go use the library to find out how to do that task.  The limitation with this is that not every computer has the same libraries, and even if they're present, the program needs to know where to find them.  So when you install a program, it searches through your computer, makes sure it can find all the libraries it might need, and if any are missing it'll add a copy of that library so it can run properly, and so future programs can also use that library.

Despite all this, there *are* programs which run from a single file.  They're usually simple programs that only do one or two tasks, where they can be all self-contained and still run efficiently.  The more complicated the program is, the more likely one of the above situations will come up.  So the complicated programs have to spend some time preparing everything so they can do all the different tasks they're designed to do.