Cheap and free to download. Most developers would rather spend weeks rebuild something for themselves than pay $20 a month for a tool.
Big AI has prompts you cannot remove. They have to because they have a big audience, get attacked relentlessly, and have to be mindful of PR events.
Now, while I can avoid the copilot/Claude code agent prompts, I am still using their models directly and subject to their prompts. Moving to use models directly is the next step, and the only way to do that is with open models. Therein, the Chinese have been building better open models, and that is why we see their usage rising.
It's more about full stack control than it is about price (imo)