To be fair, I think this is partly because ChatGPT went viral long before they had an official app. I remember thinking, "Why the heck haven't they made an app yet?" And it was notable when it was finally released. So many people had already been using unofficial apps by that point.

GPT-3 came out in June 2020, that's when a lot of developers started talking about it and early implementations using its API started to appear. The ChatGPT web UI came out in November, 2022, at which point it quickly went mainstream. By January, 2023, they had one hundred million users. They still had no app at this point. GPT-4 came out in March, 2023. Still no app. It wasn't until May, 2023 that they released an iOS app, followed by Android in July, 2023. It was a whole extra year until they released a Mac app in June, 2024. So many developers beat them to the punch on making a UI for their own product in the meantime.