It's moments like this, I'm glad I knew some Punjabi, otherwise I don't think I would have guessed all the other variants of uncles, aunts and grandparents. This is the only one where I had to look, as the Punjabi version is maasar and maasi.
I hope someone updates the actual definitions to these words.
In English, using "uncle/aunt" without a possessive pronoun before or a name after sounds strange in pretty much any context. Likewise (but in fewer contexts) the same goes for "father/mother". But in Hindi, it seems using relationship terms by themselves is much more common.
Caveat: I'm just another Hindi student like you.
Good observation. It is because most relationship terms (with some exceptions) are also terms of address in Hindi.
Calling anyone older than yourself with their given name would be considered very rude in the culture. So, you would often call your uncle named Raj - 'Raj mausa ji' as just 'mausa ji'. ('Ji' is an honorific and can be dropped if you are close to the person, especially for certain relations.)
This often carries over even in the third person. In sentences such as 'मामा आज आ रहे हैं', you have to remember that मामा is just how the person would be addressed and thought about and as a result, a possessive is unnecessary (similar to 'Dad is coming' in English).
As an aside, the practice of not taking the name of people older than you is not limited to people you are related to. Even a stranger who is older would be called something like चाचा (uncle) /भैया (term of address for your older भाई) or चची (aunt) /दीदी (term of address for your older बहन).
This is also why uncle/aunty are liberally used in Indian English for strangers.
Yes, it can get even more complicated with separate words even for dad's older brother and dad's younger brother! I'd like to note that, coloquially, mom's sister and mom's brother are not pronounced like they are here on duolingo. We say "maasee" rather than "mausee" and "maama" rather than "mausa." I've actually never heard them said the way duolingo says them.