Why? I only recently came to the realization of this habit, and I spent much time pondering exactly why I do this. OCD? Maybe, but I don't know.
After a great deal of consideration, I came to the conclusion that I think it has something to do with my fear for my numbers' safety. See, if my words were to attack the poor numbers standing at the end of the lines, the numbers wouldn't be able to do much to protect themselves. They're greatly outnumbered (pun intended), one number for every line of many words. If the words chose to attack, the numbers would be goners.
So, to prepare my numbers in the case of potential word rebellion, I give them parentheses. It could act as a sort of shield, right? At least more so than a useless period. If a parentheses is a shield, a period would be a rock. Oh, like throwing at rock at some words will do much. It might take out a few letters, but you'd still be left with lots of them coming at you.
Lesson to be learned from this?
Rocks are not sufficient defense against a horde of angry words.