How to show google.com in an iframe?
The reason for this is, that Google is sending an "X-Frame-Options: SAMEORIGIN" response header. This option prevents the browser from displaying iFrames that are not hosted on the same domain as the parent page.
See: Mozilla Developer Network - The X-Frame-Options response header
can't add google.com as src to an IFrame
Google sets the x-frame-options response header to SAMEORIGIN
Why does google.com not load in an iFrame
Because it has X-Frame-Options Header policy and browsers tend to respect those policies.
Read here and here
The "X-Frame-Options" allows a secure web page from host B to declare
that its content (for example a button, links, text, etc.) must not
be displayed in a frame of another page (e.g. from host A). In
principle this is done by a policy declared in the HTTP header and
obeyed by conform browser implementations.