The 125-word ceiling is what sends people looking. A 1,500-word essay is twelve pastes, and each fragment is rewritten blind to the others, so the connectives start repeating in a way you only notice on a read-through.
The other reason is shape rather than limit. QuillBot is a paraphraser with a humanizer attached, and people who came for the humanizer often find they are paying for nine tools to use one.
A third reason is narrower and worth naming: the humanizer covers five languages where the paraphraser covers 23. People who bought QuillBot for multilingual work sometimes find the specific tool they wanted is the one with the shorter list.
None of that makes QuillBot a bad tool. It makes it a tool with a shape, and the question is whether that shape matches your work. Our full HumanFlow vs QuillBot comparison sets its numbers against ours in detail, including where QuillBot wins.