Yep, as a rule of thumb you can think of it as a convention. It might have come from the middle English and even before that, but not to complicate things it´s better to accept that and make it a convention.
It would slow the learning if someone learning German would want to know why all nouns begin with capitals, or the numbers are written in a reverse way (one-and-twenty instead of twenty-one, or one-and-thirty instead of thirty-one)...and so on.
The learner of Spanish would (in similar way) want to know why they use the inverted question and exclamation marks, etc...
In English why "you" comes with "are" (when are means ''is" in Portuguese), or to know why "news" could mean "notícia" as in "no news is good news'', and the list goes on and on.