why ether are inert substances?

Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'.A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether" (CH3-CH2-O-CH2-CH3).

Notice only single bonds ...

Ethers in general are of low chemical reactivity, but they are more reactive than alkanes and undergo cleavage and can form peroxides..

The are explosive because of their very low flash point

They are generally inert because of the strength of the bonds in the molecule ..

I don't think they are! I know diethyl ether, the one that used to be used as an anaesthetic, is highly flammable, for example.

Ethers have a double-bonded oxygen atom coming off a carbon atom in the middle of a carbon chain, and given the electronegativity differences between carbon and oxygen, the double bond and the ability to attract further electron density from neighbouring carbons, we would expect ethers to be quite chemically active - and I believe they are.