They were but read again my reply. And besides the Forts did not go in high at first. Because fighters could not touch them because of all guns. Being Forts. And lastly, higher less targets destroyed.
…the Ju 49 was designed to investigate high-altitude flight and the techniques of cabin pressurization. It was the world's second working pressurized aircraft. By 1935, it was flying regularly to around 12,500 m (41,000 ft).
I'd say that demonstrates at least some interest on the part of the Luftwaffe.