Partial restatement on incremental by unique key

[dberardo-com]

the doc states:

Note: Models of the INCREMENTAL_BY_UNIQUE_KEY kind are inherently non-idempotent, which should be taken into consideration during data restatement. As a result, partial data restatement is not supported for this model kind, which means that the entire table will be recreated from scratch if restated.

why wouldnt you allow the user to do a partial restatement all-the-same ? say, like simply upserting rows.

I believe i am missing something probably.

in particular in my use case i have a table in which i want to perform UPSERTs, so appending new rows if none exist or simply overwrite existing ones, but without having to erase the table every time