Ideographics scripts, such as hieroglyphics, use complex characters to represent ideas. Another well known ideographic script is Chinese, which uses thousands upon thousands of characters to represent all the different ideas and concepts in the Chinese languages.
Emoji aren’t an ideographic script. While they contain some information of their own, they are not systematic, which means they don’t form part of a language, and don’t provide information beyond what their form (their picture) shows. A word in Chinese or English, or any other language, can have multiple meanings that will be understood in a given context.
“book” can mean “a series of pages with words written on them in a specific language” or “reserve a seat for a play, movie, or transportation”. The problem with emoji is that there is no commonly accepted system for determining what an emoticon would mean in a given sentence.
You could write with emoji “eye heart (finger pointing to the right)” to mean “I love you”. But here you would be relying on someone to connect the pronunciation of “eye” with the word “I”. This would not work in other languages. Not all cultures would connect the heart to love. And the finger pointing would be ambiguous. However, in Chinese you can write “I love you” with ideographs.