Because the basic but functional Irish vocabulary that Duolingo teaches uses glas for "green".
Many exercises that ask for an English to Irish translation of "green" will accept uaine if you use it in your answer (if this one doesn't, report it), but uaine isn't used in the Irish to English exercises.
You could say - Here there are thirty green cars
but "here you are!" " Seo duit" does not mean "you are here" Tá tú anseo.
In the sentence the cars are not being presented; it is stated they are "here"