Cryptocurrency exchanges don't necessarily issue these forms, which means that the taxpayer has to report the details themselves. In particular, the IRS wants each transaction reported on a separate row of Form 8949, which may be laborious for those with many transactions.
(Note that an acceptable alternative is summarizing on Form 8949 itself and attaching details of the individual transactions separately. See "Exception 2" at https://www.irs.gov/instructions/i8949#idm140393400093152)
