Why don't you have to press CTRL+ALT+DEL to logon anymore in Windows 8?

In previous versions of Windows, you had to press Ctrl + Alt + Delete to logon, or to unlock a locked workstation. This was because this key sequence was recognized only by the OS and thus other software couldn't intercept it and display a spoofed logon screen to capture passwords.

Starting with Windows 8, you now just have to press Enter to get to the logon screen.

What is to stop someone from writing a fake logon screen? Did Windows 8 add some sort of new security mechanism to mitigate this security issue?

I do not believe this was enforced by default in earlier windows versions either. There is a group policy setting you can use to enforce this.

Computer Configuration - Windows Settings - Security Settings - Local Policies - Security Options - Interactive Logon: Do not require CTRL+ALT+DELETE

disable that and you will be required to press ctrl+alt+delete.

same procedure for a domain gpo or standalone. I still turn this on in my home as a default security practice for the reasons you mentioned above.

Twisty's comment is correct, the accepted answer is not.

You can test if you like. Create a fresh AD domain and two fresh installs of Windows, one Windows 8 or newer and one Windows 7 or older. Before domain join, neither OS will require CAD/SAK/SAS. After domain join, Windows 7 will require it and Windows 8 will not. It is not due to group policy, if you do happen to explicitly apply this setting via GPO as described in the accepted answer, then you will see that all Windows versions will require CAD/SAK/SAS.

MCSA in 2012 R2