Same reason that you don't just replace your rand() implementation with "return 4; // chosen by fair dice roll". If you need randomness for whatever reason, biasing the generator is compromising quality.
In case of LLMs, you can look at it from high and low level.
At low level - if you could do with less randomness, you can always lower temperature. You usually keep it (or for SOTA providers' chat UI, they keep it) at a level where it's about right level - high enough to allow for more creative leaps and interpretations, low enough that it doesn't go off into crazy land after the third paragraph.
At high level - creativity is driven by randomness. If you had an author (fiction or nonfiction) you like for their both broad and deep range of insightful thoughts, would you be happy if they suddenly developed an acute porn obsession and uncontrollably added lewd subtext to every other sentence? Still creative, still deep, but now with that one strong attractor that biases their every thought in a single direction? Would you trust/enjoy their output as much as you did before?
That, slightly exaggerating to make it more obvious, is what "loss of quality" means here.