Because they're not fabricating any printing plates or using an actual printing press, or any technology that gets you a high quality result. A print on demand book is basically going to come out of an office laser printer, because that's the technology for low-volume printing.
Most POD presses actually use inkjet because it's less expensive. The result is much lower quality.
Why do you claim that so confidently, when many people say otherwise? Are you just going off some metric like DPI?
You're probably missing things like a sibling comment mentioned: "professionally printed books for example use slightly gray letters on creme paper." I don't think you could get "slightly gray" with a laser printer, and print-on-demand seems to basically use bright-white office paper (probably for reasons of laziness and cheapness).
Those people are fetishizing the limitations of offset printing. You simply can't produce sharp blacks comparable to an industrial laser printer with offset printing.
> I don't think you could get "slightly gray" with a laser printer
You absolutely can. But pure black on Natural Shade (off-white or cream) paper looks much better.
Most POD setups use inkjet printers for cost reasons which results in poor print quality.