For a few reasons.
First the amounts won't end up nice and round at particular hot button price points. ($5 drinks all day!) [1]
Second, the price will have to be displayed higher and in theory this will make it less likely for someone to get a particular dish. If the restaurant could get more money for the same disk in theory they would already be doing that.
Bottom line is the same way that autos are sold (or airline tickets where you now pay separately for baggage) advertising a lower price and having add on's sometimes (but not always) works in your favor in generating sales. Because you are able to price below magic numbers floating around in people's head.
[1] In the same way pricing an item at $4.99 will in general generate more sales than $5.25. (Example not restaurant pricing but to prove a point).