It doesn't, strictly speaking, and in fact, it barely does.

It's usually just a symlink from libwhatever.latest.version.number to libwhatever.so, on the theory that you probably want the latest version of the library (with all its improvements and bugfixes).

There's sometimes an intermediate symlink to the newest major versions too (libwhatever.latest.so, libwhatever.2.so), to avoid breaking changes.

It doesn't. Just don't install it if you don't want it.

Security. By having defaults the operative system can share the most secure minor version of that library (or multiple major if necessary). When you have a copy of a library for each program then most of your programs will end up using obsolete or unsecure dependencies.

Plus it also cuts down on disk space usage.