Of course there’s comparison, but yes, technically every student could get an A. On some projects, most do, but in general, not every student wants to get an A. Some look at the rubric and feel, “That’s all I need to do to get a C” and stop there on purpose. My “Generally A” lad had many students tell him, “You know you don’t need As to pass. You only need a C.” That’s just as much of a mindset as the top “driven” student mindset TBH - perhaps even more common than it where I work.
Tests don’t have rubrics. Those have set numerical numbers to get an A (93 or higher). If every student got a 93, they’d all have As (well, A- if I get technical, a true A requires a 95). No different with rubrics.
Classes are not the same. Technically they might be Bio Level 2 (our average level - generally college bound), but within the first block class you might have 2 really good students and in the 4th block class you might have 15. Bell curve grading wouldn’t suit at all. It’d be awful.