Yes, that seems to sort of be the criticism and mixed results. Although not everyone has a complete protein diet so theoretically although it breaks down the idea is you then have all the things you need, should your body choose to use it to build collagen.

But I agree, I'd rather start solving deficiencies at the diet level than the supplement level and haven't integrated collagen personally so far.

TBH I suspect marketing plays a big role. "Collagen = good, therefore just buy it and eat it" makes logical sense if you don't actually do any research first.