To expand on this, it’s the naming convention that Java defines.
Package names are written in all lower case to avoid conflict with the names of classes or interfaces.
Companies use their reversed Internet domain name to begin their package names—for example, com.example.mypackage for a package named mypackage created by a programmer at example.com.
Further it isn't always com, you'll often see net, org, and rarely even io show up as the base package. All that said with the introduction of modules and some of the other recent changes I'm not sure how much longer we're going to see the reversed DNS naming system stick around.
I addition to the answers others have given, consider the large centralized package repositories for software development: npm, PyPI, crates.io, etc. If you frequently publish packages to these indexes, you often will find name collisions and are forced to come up with clever names. In the case of PyPI specifically, it is even worse because while your pip package might be uniquely named, the actual importable python package may not be, and you end up where two pip packages clobber each other. Java's system is verbose but it is simple and solved this problem decades ago.