Hm, I suppose you're right.
However we can't use уставшая in all cases. For example, a person who is tired would rather say "Я устал!" than "Я уставший!" Especially when the word is alone (without такая etc). So I think it is easier for the Russian learners to memorize only one but flexible and always appropriate way to express it.
уставшая is a participle, maybe that's why people don't prefer it here; participles are more common in written and/or formal Russian, not so much in conversation (though there are exceptions like следующий 'next' or бывший 'former'). I have a few videos about them here (http://preview.tinyurl.com/z3obl53), but people who are just starting out with Russian shouldn't worry about them too much at the beginning.
Yikes, this is really confusing me. I can't really understand how "I am tired" and "I was tired" are interpreted the same. They're not the same - at least not too me. "I am tired" - still currently tired (present). "I was tired" - was tired at the time (past). There has to be some way of distinguishing between the two, no?
I think that "I got tired" is more correct here as a Russian talker (my family from Ukraine) ;) basically, she said something that happened to her in the past... why? because you can use that in a sentence in Russian when "I got tired yesterday after the work" It's my opinion, but that option 100% should be accepted
Confusing to use 'to be tired' so many times in this lesson. I understand that it is the past tense but in English is seems like the present tense (even though 'tired' is a past participle so in a way it is past tense in English too! Maybe think of it as 'I tired' rather than 'I am tired') It would have maybe been better had Duolingo chosen a more straightforward example of the past tense for a first lesson about this topic, no?