Why is it impossible to edit smart drums?

Hello there, lococo.

It sounds like you are trying to edit smart drums in GarageBand for iOS. The drum groove of a Smart Drum can be edited per the information found here in the online Garageband for iPhone Help Guide:

Play the Smart Drums - GarageBand for iPhone

Edit the drum groove after you record

After you record a drum groove, the grid shows what has been recorded at the current playhead position. For example, if you recorded a snare drum starting at bar 5, moving the playhead to bar 5 (or later) shows the snare in the grid.

You can edit the recorded drum groove and try out your changes without altering the recording. Playback starts when you add or move drums in the grid so you can hear your edits. You can stop playback by turning off the Power button . To keep your changes, record the groove again.

Thanks for participating in Apple Support Communities.

Kind Regards,

Pedro.