^ because they CAN!
As 75KoDe says, that doesn't always mean it's a good idea, I can imagine the scene...
"But heir Zetsche, why would we put a 6.2l 503hp engine in our slow-selling MPV?"
"Because we CAN! F*** JAAAA... MERCEDES FOR LIFE!!!"
.. and we got the R63 AMG.. that didn't go down too well, but the R-Class was dropped anyway so it wasn't that costly a mistake.
Which brings me back to my point: what is the role of the CLS now? If the GT4 would be based on the same platform (transaxle) as the GT, I would be perfectly fine.
But no, the CLS and the GT4 are based on the SAME platform, they will share the SAME engines (more or less) and they look almost identical. I simply don't get the point of this car.
As long as the public perceives it as being a sportier Porsche competitor, it doesn't really matter what's under the skin... seems that's fairly key to the German automotive industry in general.
Personally, I'm pro-choice. Some people will prefer the CLS 63 AMG, some the AMG GT4, If MB can make financial sense of it, then fair play. Clearly they're brand building AMG and basing a model of an existing platform is a easy way to do it. It's working well enough for the Maybach brand.