Because the point is to teach you to speak the language in a Romanian way. Not in an English way. Sentence formation is different. In fact English is a strange language as we do not have so many feminine or masculine words. . When most European languages do. Sentence formation is very important otherwise you sound like a fool. And to be honest Romanian sentence formation is similar to most European languages. I will give you an example of why they do this. You know when you meet people who are new to the UK. That speak English alright but do not yet comprehend sentence formation. They know the words but do not speak them in the correct order an example. Instead of saying lets go to the shop. They might say Me you go shop. Its sad but native speakers will judge this person by the way they speak. They will think the person is not very intelligent. Because the way they form sentences. Also things like how you spell your name are different. In Romania you put your last name first and your first name last.
Nowadays, almost all common keyboards have proper, yet, most common alphabetical letters... Like GBoard (the official Google keyboard), for Android users! Also on iOS, as I know, there is no problem. So I think he/she will not get into trouble... So I believe there is no need for Duo to add these characters to their mobile apps.