[nikhilgupta345]

Because there are other websites with different endings (.org, .tk, .edu), etc..

Also, what if they are advertising the website of the server that they are currently on?

Next, just because it has those endings does not mean that they are advertising a site.

It's unreliable also because other things have .com and .net in them. Although rare, it could happen. For example, somebody could type .network or .community or something like that and it would block it as spam.