Why can't humans just take in feral cats?

It would be wonderful if feral cats were safe for humans to be around. However, the fact is that feral cats are often fearful and therefore can react violently to human interaction. Unfortunately, many of these cats are diseased and are therefore capable of spreading disease. Finally, feral cats are extremely hard to domesticate. Animal shelters can't exactly take them in either. These shelters are dedicated to finding homes for animals that are able to coexist with humans easily. Feral cats don't fit in this category. Sadly many of them are often brought in and euthanized on the spot. This sad reality could come to an end if people took more responsibility in spaying/neutering their cats.