Request metering is the usual reason. A one-line fix and a 600-word rewrite both cost one of your 80, so small edits are expensive and there is a quiet incentive to batch work you would rather do as you go.
The 600-word ceiling on Basic is the other. Anything longer has to be split, and each fragment gets rewritten without sight of the others, which is how repeated transitions survive into the final draft.
A third, less often stated: the free tier is 3 requests of 250 words. That is 750 words to decide on a subscription, and it is not enough to see how the output behaves across a full piece.
None of that makes WriteHuman 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 WriteHuman comparison sets its numbers against ours in detail, including where WriteHuman wins.