"Gary Drescher" wrote

The literal meaning of 'hobby' is just as you say, and is not disparaging.
However, words such as 'hobbyist' or 'amateur' are sometimes used to suggest a
superficial level of competence, in contrast with the presumed expertise of a
'professional'. People may become defensive about that connotation.

Right. It could be that it is because we think of a hobby as play. No one
wants to think of themselves as "playing" at flying.
--
Jim in NC