Don't laugh, the question really puzzled me.

This question raises several issues around the development of cooling systems on WW2 German aircraft. The problem is that there is considerable drag associated with the required area of the radiators, so the basic idea that inline engines are intrinsically less draggy than radials is shaky at best. The annual radiator or RingKuhler immediately behind the propeller (I think that’s the correct German term) as on the Ju88A was an attempt to reduce the overall drag of the entire system at the expense of frontal area. This did make it more convenient to exchange complete power units (“the “power egg” concept”) and also greatly eased the conversion to radial engines, as on the BMW801/JU88R. As power increased so did the cooling requirements, and radiators had to become even larger. The final German approach was the Drumkuhler, where the front opening led to a ducting so that the actual radiators were not face-on to the direction of travel but parallel, reducing the overall frontal area and hence drag. These could be seen on the Fw190D, Ta152 and He219, amongst others. The final designs of German piston-engine fighters including the Me209 adopted this system, whereas the Allies went towards greater us of ducts to large radiators inside the fuselage or wing – most notably the P-51.

Why the liquid-cooling piston engine mounted on Nazi combat aircraft was always air-cooling piston engine alike?
Even further, don’t you think this one was like a propeller-fan driven engine more than a piston engine? Why?

Not “always”… far more German WW2 in-line engines had “normal” in-line cowlings and radiators than had circular cowlings with internal radiators. Look at the entire Messerschmidt line, most Heinkels, and so on.

Are you refering to the annular radiators fitted to some aircraft fitted with inverted inline v12 engines such as the Focke-Wulf Fw 190D-9, Focke-Wulf Ta 152, and some Junkers Ju 88s.

Do you mean why were the engines upside down, or are you talking about the fan cooled FW190?
Can you post a picture?

Pete