RNAses in general are rich in Disulfide bounds, which grant them extra stability in adverse conditions, such as heat. That's why some RNA extraction protocols include 2-Mercaptoethanol in its early stages (specially for samples rich in RNAses). The 2-Mercaptoethanol is a reducing agent capable of reducing the RNAse's disulfide bounds, rendering it more susceptible to the other denaturing agents.

The answer may be, in part, that the RNase is able to refold into its native structure after heat denaturation. This is true of many small proteins.

Thermophilic bacteria make proteins that are inherently heat-stable, requiring very high temperature to denature, so nature has already solved this problem.

Daniel,

RNase should not be a concern under normal extraction conditions with strong denaturants.  Why are you concerned?

RNases are active on the skin, which is a pretty adverse environment for an enzyme. It means that a RNase needs to be tolerant of dehydration and changes in salt levels among other things. Perhaps, as a side effect, these robust characteristics also protect it from heat?

RNAses in general are rich in Disulfide bounds, which grant them extra stability in adverse conditions, such as heat. That's why some RNA extraction protocols include 2-Mercaptoethanol in its early stages (specially for samples rich in RNAses). The 2-Mercaptoethanol is a reducing agent capable of reducing the RNAse's disulfide bounds, rendering it more susceptible to the other denaturing agents.