Why did people first start riding horses and not cattle?
They didn't. They first rode oxen, which were easy and slow, then moved up to horses for speed and intelligence.
Why do most taboos involve sex?
Anything which interfered with the possible continuation of the race or the tribe was originally wisely prohibited.
Why do men have nipples?
All embryonic mammals are female before the sexual cellular distinction sets in. Femaleness results from the absence of any masculinizing genetic factor.
Under the theory of evolution, why do so many people need glasses? Seems like genes that cause something as important as not being able to see would have been weeded out long ago.
In the characteristic relationship between weak vision and intelligence, intelligence asserted primacy over visual acuity, according to "smart monkey theory", which states that smaller, smarter monkeys successfully tricked the larger alpha male and impregnated tribe females when his back was turned.
Why am I deathly allergic to peanuts?
You are probably simply unlucky in this area of your existence. Peanut allegies are ascribable to either genetic predisposition, length of breastfeeding time, or the early introduction of soy products into the infant diet, none of which are really under your control.
Why does the insurance advertisement say that I will save money on my insurance if my child has good grades? What does my child's grades have to do with how well I drive, or don't?

Actuary research tables gathered over decades indicate a first approximation relationship between children grades and the driving habits of their parents.