Maybe they want to have their own protocol and standard for file editing for training and fine-tuning their own models, instead of relying on Anthropic standard.
Or it could be a sunk cost associated with Cursor already having terabytes of training data with old edit tool.
Maybe this is a flippant response, but I guess they are more of a UI company and want to avoid competing with the frontier model companies?
They also can’t get at the models directly enough, so anything they layer in would seem guaranteed to underperform and/or consume context instead of potentially relieving that pressure.
Any LLM-adjacent infrastructure they invest in risks being obviated before they can get users to notice/use it.
I built my own simple coding agent six months ago, and I implemented str_replace_based_edit_tool (https://platform.claude.com/docs/en/agents-and-tools/tool-us...) for Claude to use; it wasn't hard to do.