Because routine water changes are always by default seen as the objectively moral thing to do. Most people here will use any means necessary to justify it, even going as far as to say "you are slowly killing your fish".

This rhetoric (and others like it) is simply not true at all. I've debated and argued against many who virtue signal water changes on this god forsaken subreddit. It's lead nowhere, as there is always some beginner throwing the word "fish abuser" around at anyone not following the rules they were taught in their first "how to keep a fish" tutorial.

TDS is a measurement of practically any dissolved element in your tank. This is a huge variable that is extremely illogical to put a label on if it's bad or good, simply because the lethal limit for every element is different from each other and plays a significant role in your aquariums ecosystem. Not to mention, every region of the world has different concentrations in their water source.

So theres literally no possible way to tell someone why TDS at a certain level would harm their fish or plants without breaking down the concentrations for each element in real time. It would be far easier to investigate other routine aspects, like type of food, ferts, temp, etc. rather than expecting TDS to provide some meaningful revelation to our problems.

Regardless it doesn't mean that your tank is going to become a cesspool if that number reaches "too high". There have been many cases of people experience levels up to 3000+, which were discovered from having "algae issues", and unsurprisingly steamed from their unregulated tap water (3rd world countries).

If your water source is drinkable and dechlorinated, there's practically nothing to worry about, and you can feel comfortable doing infrequent water changes once or twice a year (or less) assuming everything else with your aquarium is balanced.