How come people cannot speak as complex as well as they write?

I feel that people nowadays are lazier than ever when it comes to define and accurate terminology. Speaking in one way, to communicating, and the other, speaking ones heart, thus, using different vocabulary to express themselves. Speaking though, is so much different to writing because, we take our time to think about the subject. When we speak, we speak slang, cutting off full words and use its abbreviations, people do understand its meaning, but it’s not proper.