Answer:
because it is easier on the eyes than a large block of text
Explanation:
Advertisers must reach out to all types of audiences and they know that some people find it difficult to focus on large text and other people because of the day-to-day running, they will not have time to stop and read an ad in a large block of text. In order to serve this audience, advertisers prefer to make their ads simplified in short, accurate texts that can be quickly viewed and understood by all.
