It's possible but its really unreliable. A device trying to reach out to an app on your phone to proxy the data while your phone is sleeping/app not running just doesn't work that well. You don't want to have to open the app while using the device, you just want all the data to be there when you look in a week.
These devices almost always have wifi since the chips usually have both anyway. And reaching out to a fixed wifi is so much more reliable.
And then the user goes out for the day, opens up the app, and wonders why the last 3 days of data is missing. Meanwhile the chip that does Bluetooth also just has wifi bundled in. Aside from the security risk, directly connecting to wifi is a vastly superior experience.
How much data can a toothbrush collect? Surely just a few hundred bytes per brushing session. The ESP32 has 160 kB of usable RAM out of the 520 kB total capacity. Surely enough for weeks of data even if the data structures are badly designed.
The problem is you can't reliably collect it over bluetooth. I doubt iOS allows apps to just run always active background tasks to fetch data over bluetooth while the app is not open.
The user expects the data on the app to always be in sync. If they check it outside of bluetooth range and see days of data is missing, it'll look broken.