Some musicians DO call as well, and some callers get out a whistle and join in once the dancers have been prompted through a couple of turns of the dance.
But I prefer a caller who has their attention on the dancers.

I find it very hard to give proper attention to other sets if I'm dancing (to make up numbers) in one set. Not a good enough musician to play, even if not calling.

Our club callers are helped to learn to call ( one dance this week, repeat it next time, better, and add another). It is surprising how a dance that works perfectly well at home goes wrong first time out and the mistake is invisible till you look again at home - then obvious.But if you are learning among friends there is tolerance.
Calling for non-dancers is different again. Easier dances but clearer explanations needed. But if calling for groups who seldom dance you can call fewer dances that you know without the card - I agree that you need it for club dances (though the card could be on the music stand if the musician knew the tune well). By the time I have rehearsed a club programme I have the dance almost memorised (soon forget it), so hardly need the card after the walk through.

I suppose a caller/musician needs both dance and tune memorised as eyes need to be on dancers.