The internet is older than the world wide web, and that’s just one service that exists.

Having a server directly behind the domain like example.com originally wasn’t common. Back in the days, you might want to make an FTP connection or a pop3 connection, or telnet or gopher, which would typically use like ftp.example.com, mail.example.com, telnet.example.com, gopher.example.com. None of these examples are web servers. You wouldn’t get a webpage back from them.

So when the world wide web became a thing, it was common to set up www.example.com and serve webpages from that subdomain.

The thing that changed in the mid 90s as the internet started to become more popular was that the world wide web quickly took over as the main thing people used when connecting to sites for information or data, so after a while it became common to have the web server respond to requests sent to the bare domain itself. (We’re ignoring ports for the sake of simplicity)

Although that change meant that you could go to example.com for most sites, or www.example.com for most sites, people had already learned that “www dot something dot com” meant a website, and so many people typed www even if it wasn’t necessary. And because there was no harm in having this set up, the tradition continues to this day to set up that subdomain and typically point it to the main web server.

But there’s no reason you have to. You can set things up so that requests to example.com get a different website than www.example.com. But it will confuse your visitors, so most people don’t.

So to answer your question: We stopped needing to say “www” in front of websites URLs by the mid 90s for the most part, but it stuck in the ‘common knowledge’ and is still around today because most people have no clue how it all works. In other words, your question is partly wrong, but the part that’s right has been true for almost all of the public internet’s history. :)