Well, then share some examples to the contrary!
Anyway, it's true that there is nothing in the Polish language itself that implies which mother is implied, it's all in the context. But I feel that Polish speakers use "my mom" less often than English (especially if it's an object in the sentence) and let the the listener figure it out themselves.
Given the fact that "matka" is really quite formal (while 'mother' is more formal than 'mom', but still pretty neutral), we want the default English translations to be "mom" for "mama" and "mother" for "matka".
"mother" is still accepted, of course. Actually it's a choice of Duolingo programmers to accept it automatically whenever "mom" is accepted.
Angielski używa zaimków dzierżawczych (mój, twój, etc.) o wiele, wiele częściej niż polski, niemalże zawsze. W sumie słowa takie jak "mom", "dad", "grandpa" czy "grandma" to mogą być niemalże jedyne wyjątki, gdzie nie jest to totalnie obowiązkowe. Można tutaj odpowiedzieć "mom".
Ah, I see. Grammatically, you definitely need a way to distinguish between the person who is being asked and the thing you're asking for. I can see how 'proszę coś' might seem logical, but the accusative is already used for the mother, so you either need a different case for 'komórka' or a preposition. Well, and Polish chose the preposition to tell them apart.
If I'm not mistaken, it would be "Le pido a mi madre un móvil" in Spanish, where the recipient of the request is distinguished by the preposition 'a', whereas the object (móvil) doesn't need one anymore. So, it's the other way round here.