Because incrementing integers require a singular central node to track the universal state and even so still have a risk of collision bucause they are not guaranteed to be unique in absence of an externally carefully managed master incrementer.
Applications use uuids to avoid colliding with themselves. Uuids exist so that web-apps can create objects client-side without waiting for a database CREATE, applications can be built with multi-node dbms, operating systems can name hardware compenents.
