Because periods are very small and a capital letter makes the semantic delineation more distinct, like indenting code does. Because capital letters are mostly

Because periods are very small and a capital letter makes the semantic delineation more distinct, like indenting code does. Because capital letters are mostly omitted when the writer is in a hurry and doesn't care very much for details, which is insulting to the reader in a context where care and attention is expected. And if you hadn't thought of those reasons - the fact that everyone else does should be a clue. Chesterton's Fence and all that. Show some humility!