Because DeepL's quality, unfortunately, is still miles behind ChatGPT. Especially when the target language isn't English.
I can read some Japanese, so I know it when it's very off. It often translates thing into totally opposite meanings, or omit entire sentence within a long paragraph. I trust broken results in Google Translate more than DeepL's when it comes to Japanese, as it's at least more literal.
DeepL also has an infamous issue where when you live-update your input (by adding more sentences), it will repeat the same sentence over and over again. You have to restart from scratch to avoid this issue.
I switched from DeepL to ChatGPT the moment DeepL introduced a paywall after some usage. But honestly, I really liked DeepL, it always worked well and far better than Google Translate for my use cases.
A model built for translation and nothing else isn't going to get distracted and wander off like a general purpose LLM is prone to doing.