Why does teeth bleaching ruin tooth enamel?

It does not. There have been many studies done on the safety of bleach on enamel and not one has proved it is permanently harmful. (yet?).
For prolonged period. Of bleaching can weaken the enamel. Carbamide peroxide at 16% or below is acceptable. Avoid very low ph of some bleaching agents because they can demineralize the teeth. Bleaching or teeth whitening monitored by dentist is safer so he can also check the condition not only of the teeth but the soft tissue as well.
It does not. I have successfully bleached hundreds of teeth with no damage to the enamel. The only side effect is occassional gum irritation or tooth sensitivity.

Related Questions

Does teeth bleaching ruin the tooth enamel?

No. Bleaching is an acceptable process that causes teeth whitening. In some cases it can cause mild to moderate sensitivity for a brief period of time, which can be managed. It does not ruin tooth enamel. Read more...
No. Just don't overdo it. Your teeth will only whiten to the extent that the enamel will allow. The color of the dentin underneath will still shine through and cannot be lightened by bleaching. Likewise, if you have recession, your exposed roots will not lighten and should not be bleached. A common side effect of whitening is sensitivity of the teeth, which is transient. Read more...
For prolonged period. -yes. Carbamide peroxide at 16% or below is acceptable. Bleaching or teeth whitening monitored by dentist is safer so he can also check the condition not only of the teeth but the soft tissue as well. Read more...
No. Bleaching your teeth will not ruin the enamel. However, if you overdo it, you may increase the sensitivity of your teeth since the bleach material opens up the tiny pores in the teeth, which allows the material to whiten the teeth. Most of the time, the sensitivity is transient. Keep smiling ! Read more...