How come so many karate people still train tai chi?
The answer is that they probably do not realise that they are not really learning tai chi.
The karate stands in the way of any real, meaningful progress.
The second answer may lie with their choice of tai chi style/approach.
If a karate person studies sport tai chi and learns competition forms, then the extended stances and exaggerated performance aesthetics may not jar with karate.
However, if they encountered a very internal, small circle style of tai chi, it may be another story altogether.