The main reason is that it was not mandated by the Prophet; in fact, he prayed with his wife and daughter there.
Also, (a) because to do tawaf you have to go all the way around the Ka'bah, and that is difficult to do with segregation, similarly it might be challenging to arrange gender segregation for sa'i between Safa and Marwah, and (b) perhaps it could cause hardship for people travelling together.
While it is true that there are some people with sick minds who misuse the arrangement there, by and large, I think most Muslims behave themselves properly there and respect the sanctity of the place, and the rules of ihraam (forbidding certain things between genders) also provide extra incentive to behave correctly, even if they may run into each other by accident.
That being said, wearing an extra outer garment can help provide more personal space without inconveniencing others.
Woman is granted the high human dignity just like man or even more if her piousness is more than him. There is no question of restriction which disturb her life, but discipline which keeps her safe and respected and keeps the man safe and pious. The Islamic life aims to keep the female as respected and honored as the noble man.
Glancing at other gender usually has a reason. If it involves lust, it will be definitely Haraam ( forbidden).
If glancing is for other reason which is completely away from desire, it should be avoided as much as possible.
- All rules in Islam are based on the benefit of human beings and avoiding them any type of harm.
- Watching opposite gender should not be a cause of harm under any circumstance.
- Men playing football or women playing any sport are been done with dress which is not necessarily according to Hijab rules, that is why watching opposite gender playing should be avoided especially when there is a possibility of harm. If all conditions of Hijab are fulfilled and there is no possibility of any harm, then it will be allowed.
The main purpose for attending places of worship is to worship, and not to mingle and mix with people of the other gender. This means that we need to keep the atmosphere in mosques and Islamic centres as spiritual as possible. With mixing between genders, this might lead to unwanted outcomes that would go against the ultimate purpose of places of worship.
It is also a matter of convenience for both genders.
This in no way does it mean that females are to be left out of any active participation in programs, or for them to feel marginalised. Rather the opposite. In most places of worship, our sisters are actively contributing to various religious and social programs. However, we know as Muslims that we do not accept "free mixing," and therefore we must preserve the moral and spiritual purity of our environment.
There are times when women must be on one side, and men on the other, and there are times when brothers and sisters can be together, keeping it within the shar'i boundaries of Islam.
And Allah knows best.
Even with Hijab, it is more respectful for females to avoid mixing with men unless there is a need. Islam focuses on all measures which keep the modesty and respect of the females.