Transactions are "hard and slow". Distributed transactions are hard-er and slow-er. Random 32 bit integer additions are likely "good enough" and are fast and simple.

Also, assuming that their storage is directly some FS, you don't have access to the transaction. You can't do a distributed transaction against something that, as a black box, does not appear to do any transactions.

In this case, doing a distributed transaction would mean to use an entirely different storage solution. A transactional one supporting TPC or similar. Magic counters sound good enough yet? ;)