Czemu ta forma nie jest poprawna
On Fridays, my dad is always buying fresh fish from the market.
The simple present expresses a habitual, state-like and stable situation. It denotes an established "fact of life" and as such is presented by the speaker. In other words, present habituals (and generics) belong in the "structural world knowledge" or "necessary reality" that obtains now and can be predicated in accordance with the speaker's "general world knowledge."
Whereas the present progressive also names a state-like situation, the state that the progressive yields is not part of the speaker's "structural world knowledge." It is not derivable from it; it is neither logically expected nor necessary, and it can only be experienced by sensory observation by the speaker at the time of speaking.
Most, if not all possible readings that the present progressive can bring about (present on-goings, temporal validity or temporariness, modal effects of unexpectedness, atypicality, surprise, irritation…) can be derived from this most basic meaning of the present progressive: present observation of something that hasn't been established in the speaker's world knowledge.
A simple observation by the son at the time of speaking is in no way indicative of the father's always-on-Friday habitual shopping for fresh fish.