Very disappointed after seen 10 seconds only option for always on display, why xiaomi?

Because.
1. a very cheap matrix is ​​used (this is not the top segment of phones).
2. not optimized software.
3. there is a bunch of phones that are outdated every half a year, a year. do you think they really need to bother with optimization?

If you really want to see what kind of AOD it is, then you can either put magisk (root), for it there is a module that allows you to enable AOD for permanent operation.
Or you can try to install an older version of the AOD application, you need either version 2.4 or 2.6. You can download it on apkmirror.
When using AOD (always on), the phone is discharged by ~ 20-25% overnight.
The battery consumption is ~ 2-3% per hour, the phone just lies on the table, it is not used for anything, just waiting.