Hi Lucy. Because you enter something in a diary. So the diary is the actual book (or online) whereas the diary entry is what you write in it

Simon's answer is good. I will add, it's like saying "write a newspaper article" - if you don't say "article" here, they will think you want them to write an entire newspaper. It's also like saying, write a book chapter. If you don't say chapter, it means write the whole book. It's also like saying, wash this car window. If you don't specify window, it means wash the whole car.

In this context, "entry" refers to an individual record or portion within a diary, so "a diary entry" specifically highlights a single, standalone log within the overall diary. This terminology is common because diaries often consist of multiple entries, each reflecting a specific date, event, or thought. Saying "a diary entry" instead of just "a diary" distinguishes one recorded moment from the entire collection of writings in the diary. "A diary" refers to the whole book or collection, whereas "a diary entry" is just one segment within it.

"write a diary entry" = "enter something in a diary"

The term "entry" here refers to a single day's writing in a diary, not the entire diary. By saying "a diary entry," it specifies that you only need to write about one specific day in the diary, rather than creating an entire diary or multiple days' worth of entries. It focuses on just one short piece of writing, like a snapshot of that day.