Malwarebytes does NOT stop Windows Defender. What is happening is Microsoft knows some people want to use alternative security solutions. But they also know there are some alternative solutions that don't play well with other security solutions.
In the old days, it was common for conflicts to appear when two security programs were running at the same time. I like to say it is like two dogs guarding the same bone - each wondering what the other is up to. While less common these days, it still can happen, but often it is just a waste of resources.
So when a third party security solution is installed, that program registers itself in Windows Security. That tells Windows Defender's real-time scanner to step out of the way. So that is what is happening here. Malwarebytes, as a third party security solution, is registering itself with Windows, so Windows Defender is stepping out of the way.
HOWEVER, Malwarebytes and Windows Defender play really well together and don't hog resources.
So you can tell Malwarebytes not to register itself in Windows Security.
1. Open Malwarebytes (right click on System tray icon and click "Open Malwarebytes"),
2. Click the Gear icon in the upper right,
3. Click the Security tab,
4. Scroll down to Windows Security Center,
5. Move the "Always register Malwarebytes in the Windows Security Center" slider to "Off" (the left).
Now both Windows Defender and Malwarebytes Premium will happily run together without interfering with each other.