One of the most important rules of storytelling is that no matter how fantastic the world is, humans are still humans. If it's a choice between changing Freeman into a cartoon character or changing the way he progressing through the game, it has to be the latter.
The humour partly comes from hearing the reactions of this established version of Freeman that Ross has created. How he would react to these situations. That has to stay consistent. As soon as Ross decided that Freeman can do a pull-up, the decision was already made. This is just a more sophisticated pull-up.