The verb 'to be' in the Present tense (which is є in all forms) is normally omitted in the 'X is Y' sentences.
But not in 'There is'-type sentences, where you use є.
Greetings ehsan - many of the verbs in Ukrainian are assumed. To be precise, many of the words themselves are assumed. For example, if I were to say "тато тут", that directly translates to "dad here". However, the "is" in that sentence is assumed, thus making it "dad is here". I don't know if there is a specfic reason for this, but that is how it is. Hopefully this was helpful.