You can do it on your own systems all you like, but the thing people love about domain names is that anyone who types in a domain can get to that particular site. The fact that we all generally agree where a domain points to is part of what makes them useful.

If I decide I want to have my personal website be called google.com I can edit my hosts file to point google.com to my webserver and everything works for me, but anybody else going to google.com is going to get Google's page instead unless I somehow convince them to edit their files or use my weird DNS server.

Even if I come up with some great reason that makes people want to accept that google.com is better when it goes to my website instead, what happens when someone else decides they should be able to use google.com for something else and they convince a bunch of people to use their weird DNS server? We could fragment the DNS all over the place and have several conflicting entries for every popular domain name depending on what DNS server you're using but is that really better?