They don't because it's practically impossible. Besides, when Chinese people speak fast the tones usually flatten out as well. You might notice this when someone is talking about a specific subject. This subject however IS pronounced very clear and with correct tones to make sure that there is no confusion. When you listen to some conversations it's easy to notice. The same happens in many languages though where common stuff is usually just said in a lazy kind of way and the important part is pronounced correctly to avoid confusion. Just imagine having to say 对 the same way all the time. Doesn't happen. When I started out I kept wondering about this because the teacher I had kept stressing us to use the correct intonation. But as soon as I started listening to some real dialogues I noticed that it's very common to just even it all out, because most of what you're saying is context-based anyway. You also have to realize that there a certain sound can have dozens of characters attached to it, so even pronouncing it with the right tone won't ensure that you say just one specific thing. Context is everything. :P

Indeed the tones are not preserved or respected in Mandarin songs. However, in Cantonese songs the tones are always essential. For example, the characters with high level tones can only stay in "So"(of do re mi fa so...), while the relative "height" of other tones are also considered.

genau...so maybe sing chinese songs r easyer than speak chinese :p

Harregarre's answer is right. When we are singing, we don't use the 4 tones of Chinese pronunciation, just follow the music, naturally.