It's not a stupid question.
What you're asking is, why does Apt only install software at a system-level, which requires superuser privileges? Why can't it optionally install software at a user level, requiring no such privileges?
The short, glib answer is that that isn't Apt's role. Apt's role is the management and installation of software that forms part of your system. If you want to install software at a user level, you can do that yourself, but you don't use Apt for it. You can compile it yourself, or grab a statically compiled binary from somewhere, or using something like flatpak. Or spin your own method for deploying.
The role of Apt
Ubuntu is a Linux distribution. A Linux distribution is at its core a collection of software from many different sources, all which have been collected into one place by the distribution's managers so that you can install a complete, working system from it.
For this to work, the distribution can't just make software available completely unmodified - for each piece of software that forms part of the system it has to "install" it in a particular way into the system so that it works correctly with the rest of the software on that system.
Distributions like Ubuntu maintain repositories of software that comprise this system. The repositories consist of many packages, which are discrete parts of the system that can be chosen and installed separately. Each such package lays out the software inside it in a way that allows it to be installed into a working system, so all the software and supporting files are already in the directory paths they will reside in on the target system. In addition to this the package comes with install, configure and/or uninstall scripts that perform any other work required.
The laying out of the files inside the package, and the install/configure/uninstall scripts in the package, all assume the package will be part of a whole system. Thus, it's not easy to take a single package and install it somewhere else, because the scripts and the layout of the package won't have been designed for this.
On top of this, the packages in the system frequently are relied upon by other packages. And, if the software in a package is relied upon - is a dependency of - another package, it has to be placed into its correct place in the filesystem and in many cases the install and configure scripts have to have been run to ensure it's installed correctly.
The tool on Ubuntu and Debian which install packages and run their install/configure/uninstall scripts is called DPKG. But, the tool that finds packages in the repository and ensures that all packages that are dependencies of other packages are also installed is called APT.
As you can see from this, every part of the software distribution stack is designed so that software is installed into a predictable location so that it can work as part of the system, and so that software can depend on other software to be there, and so on.
Operating systems like Ubuntu draw a very clear distinction between software that is provided as part of the system and software the user has installed themself. This distinction is even built into the Filesystem Hierarchy Standard (FHS) which dictates how Linux distributions (and unix-like operating systems) lay out their software. Software that is installed by the user is to go into separate directories on the system, such as
- /opt - for system wide software that isn't part of the OS but is provided by other vendors
- /usr/local/* - for system wide software that was installed by the system admin manually, that isn't part of the OS
- home directories - for software installed by end users in their user account, not system wide
In addition to the above, other directories can be used if there is justification for making up your own directory scheme and not using the above.
What other options are there?
If you write software yourself or you grab it from an upstream source and adapt it yourself, you can modify your software to be installed where you like and work how you like. So that's always an option.
If you want to use your operating system's (or another Linux distribution's) packages but install them in another location, various hacks exist to get software out of a package and do this. But, the extent of success of doing this will depend on how the package is written, how its install and configure scripts work, how dependencies in the package work and so on.
You can use containerisation (with chroot) to install into a directory, and even run a separate copy of Apt in that directory. Then, you have basically a cut-down Linux distribution in its own right inside a directory within your OS.
A technique like this is what is employed by technologies like Snap or Flatpak, and is why they are pretty flexible in how and where they can install software.