That's pretty logical, actually. An analogue of "was" means you are talking about the Past, "will be" refers to the Future and the lack of any verb is interpreted as Present.
I mean, seriously. English does it all the time: the "-s" ending is used in "He eats", you should use -ed in "I watched TV" but no ending in particular is used in "I see you" or "You eat too much". The lack of a certain feature might be a meaningful feature of itself.
Oh, it's like:
Моя сестра (noun subject) вчить (verb predicate) українську . //My sister learns Ukrainian.
Моя сестра (noun subject) - студент (noun predicate). //My sister is a student.
But dash is not necessary in:
Моя сестра (noun subject) хороша (adjective predicate). //My sister is good.