"Instead, OJ could allow a simple line like showAwesomeTweet('2131221121'), which would be guaranteed to work everywhere, simply."

Ah, but you are sacrificing uniformity of design there. What if I would like the style of the tweet to be consistent with the overall style of my app? And this style is managed by a separate designer not a programmer?

Modularity works fine for behaviour, because it is better to abstract and encapsulate behaviour. But modularity doesn't necessarily work great for looks and feels.