Edit: Incorrectly answered with information about the Starship vehicle. ErinAnne's answer here correctly answers the question which is about the Super Heavy first stage. Please upvote that one and if required re-assess your vote here.
The updated Environmental Assessment for the first three Starship/Superheavy launches is here.
While it does not give a specific reason for hard landing the Starship a repeating thread through the document is:
- That all debris will quickly sink and not become a hazard to navigation
- That any impact explosions will be small enough not to harm sea life
The current plan ensures both by doing a terminal velocity impact following re-entry with the minimal propellant load for stability (specifically calls out venting down to this amount on page 6).
A full successful soft landing will produce a navigation hazard that SpaceX would need to plan recovery and disposal of. A failed soft landing might impact the ocean with a much higher fuel load and resultant explosion.
Given the concerns around re-entry damaging Starship SpaceX may have chosen this route as one that is easier to get approval for since a 'damaged in re-entry' produces results pretty close to the planned one, and avoids needing to hire a large recovery fleet.