Why are employers asking for w2's

They may be asking for proof of previous employment and verification of salary. I know that when I have done background checks on individuals and the company was out of business they requested W2's as the proof, because the company could not be contacted.

They generate the W2s in the first place and should have retained a copy.

Is the government auditing their timely forwarding of income tax withholding or social security contributions?

Or are the employers you are referring to wanting W2s that were produced by prior employers as proof if the experience you listed on an application?