What I don't understand is the need for this to be codified. How often does it really matter what the exact carrying capacity of a mule is? Do you really need the exact movement rates? Why?
Again, it depends on the situation.
Here's an example. Let's say you have a scenario where the PCs are in charge of logistics for an army, and need to muster up enough mules, horses, etc. to bring X amount of goods from point A to point B in a certain amount of time, given limited resources. Then knowing things like how much a mule can carry, how far the mule could go in a day, how much food the mule would need, how quickly a mule can be trained, what options are available to improve any of those characteristics (e.g. spells, training techniques, equipment etc.) WOULD be very important to solve the problem.
Now, admittedly, there are a few solutions. You could just make up the information you need. But that would require a significant amount of work to get right, and if the players came up with an idea you didn't think of you would have to create even more information on the fly. More likely what you would do in such a situation is abstract the whole thing out into some sort of skill challenge. But that has a few problems:
1. You have to know in advance that that skill challenge is coming so you can prepare for it. Admittedly in the example above, it's probably not something that would happen unless it was planned in advance. But it's easy to imagine scenarios that aren't.
2. More importantly, it reduces the opportunity for meaningful decision making on the part of the players. Let's say the players are deciding "Should we train more mules now, or are we better off spending our time collaring the oxen and forcing them into service?" Presumably in a skill challenge, the player would say what they wanted to do, and then roll a skill check. The only link between the player's decision and his chance of success is a DM judgement (as to what skill, and whether to give a bonus or penalty) - so the player is essentially trying to guess the DM's judgement. But if the player had the stats, they could actually make the decision based on the stats, and thus they would have an opportunity for their problem-solving skills to make a difference.
(For an analogy, consider combat. The rules don't tell the players to just describe their tactics and the DM gives a bonus or penalty based on his judgement; the rules give the players tools to actually implement their own tactics, and then the rules determine the result. Having more rules ofr mundane items would make it so you could extend that paradigm into non-combat problem solving.)