There are two kinds of decrees of shari'ah:
1. The decrees and prohibitions that come directly from Allah; we call them “taabbudi”, that is to say, their reasons (wisdom) are not known.
2. The part whose reasons (wisdom) that lay on the divine decrees and prohibitions may be searched about; we call them “maqul al-mana”.
We can look at your question from a different aspect. "Why is the morning prayer four rak'ahs but not ten or twenty rak'ahs?" The answer is "because Allah commanded us to do so."
The noon prayer was determined by Allah as 10 rak'ahs. To search the wisdom behind it will turn out to be in vain. The real answer to the question is because Allah commanded so. However, some shari'ah rules may be explained with wisdom, but they are not the real reasons. The actual reason is Allah’s command and prohibition.
For example, "Why did Allah command salah (prayer)?" One can list the wisdoms and purposes of it in volumes of books. One can answer why we fast by searching its wisdom and reasons. However, reasons (wisdom) and benefits cannot replace Allah’s decrees. For example, one of the reasons of fasting is to empathize with the people who suffer hunger and approach them with compassion.
So, a person can say “I will experience more hunger so that my feeling of compassion can be increased and I can help the poor much more.” Although the time of imsak (the time to begin fasting) is at 4.00 a.m., if one intends to start fasting at 11 o’clock at night, but if he breaks his fast 5 minutes before iftar (the breaking of fast at sunset), can his fast be acceptable? Certainly not. There is a determined time to break the fast. And although that person stays hungry longer, his fast is not accepted. That is to say, the wisdom that required for fasting is applied (by staying hungry), but because of breaking the fast at a time that Allah does not permit, his fast is not accepted.
Thus, we should look at the all of the decrees and prohibitions in this way. That is to say, we do it because Allah ordered us to do so. Certainly, there may be reasons (wisdom) behind it, too. And these reasons can be searched. It is also knowledge and worship. However, wisdoms and benefits are definitely not the actual reasons, but details.