Because it's an option already. "Transliterate to Latin letters."

Edit: I should add that you should look at how many keyboard layouts there are. It's kind of silly that for me to use an OSM based map and go to any county east of Slovenia I need to both have the keyboard AND know the transliteration of the alphabet.

Have you seen the Armenian or Georgian alphabets? What makes the K sound?

Did you know every dialect of a Slavic language using Cyrillic has it's own distinct keyboard varied by mostly the letter for the nya sound and J?

Greek?

All while transliteration works fine in Google.