Because there's a good chance down the line you will need to do some sort of range query. Let's say you want to add and backill a column. Not too bad, you create a partial index where the column is null and use that for backfilling data.

But at a certain scale that starts taking too long and a bigint column would be quicker. Or you decide you need to periodically scan the table in batches for some reason. Perhaps to export the contents to a data warehouse as part of an initial snapshot.

You can skip enumerating these possibilities by having a bigint surrogate key from the get go. There's other advantages as well like better joins and temporal locality when the bigint index can be used rather than the uuid.