I have installed my own dishwashers since the first house I bought, never seen one that had an electrical plug connection. Nor have I seen one on a garbage disposal for that matter, both are always hard-wired in my experience (I'm in the USA). I had assumed it was a code requirement since I'd never seen it done any other way.

For apartments, where tenants are more likely to abuse the appliances, a plug-in might make some sense, as it would make installing a new dishwasher slightly easier, and avoid needing to cut the power on that circuit to wire up a hard connection.