If you want to go for an insert-only approach, there is a way, but it also involves some overhead. You can do a left_anti join of your new data against the delta table.

Only records that do not yet exist remain and those can be appended.

Of course this also means that changes of existing records are not registered in the delta table.

The reason it works as it is is because parquet files are immutable. If you want to change 1 record in a parquet file; you have to rewrite the whole file.