Tends to depend on the exact implementation, some of the more self-timed one aren't discharging/charging if the cell isn't in need of that occurring. Micron QSPI NOR for example complete a lot faster if the blocks/part are already blank.
ST doesn't allow for multiple knock-down writes, where you zero out the cells before erasing, so the charge levels are equalized, a technique used to reduce "memory" / ghosting, where prior content persists at some much reduced level.
In ST's designs a lot of the wear/stress seems to be with the charge-pump and the life there with the stresses from higher voltages.
@STOne-32 any white papers on the various flash arrays, geometries/processes?
Probably laziness or simplicity. Now, with much of my update code I tend to explicitly blank check before doing an erase, but this is more to improve the perceived speed of operation, a read/check is very fast, and will terminate early if content seen, whereas erasing usually takes an age, especially with larger blocks/sectors.
The ECC methods may imped the ability to check erased/virgin state on flash lines.
