How come my host file doesn't contain IP address of my frequently visited websites?

I did vim /etc/hosts and my hosts file looks like:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost
fe80::1%lo0     localhost

How come it doesn't contain the ip addresses of my frequently visited websites like stackoverflow.com, youtube.com, google.com, twitter.com, etc.. ?