Security features improve. At this point, the rate of convincing fakes of DLs in the Real ID program is miniscule. If you buy a fake, it will most likely be an older model. Assuming a 10 year cycle, 10 years after Real ID went into effect, no ID can be valid without the new, much-more-difficult-to-imitate security features.

Some of these are physical (laser etching instead of dye printing) and some are internal controls. IDs are now printed at central facilities per state which meet stringent physical security standards and apply the 2-man rule, rigorous auditing, and other controls to prevent the aribtrary printing of IDs by employess. A "friend" in a local office used to be the best way of getting fakes made. That loophole is now closed. Equipment for serious laser etching is too expensive to buy without serious capital and much more difficult to explain away than more innocuous-seeming pigment printing tech. It is also harder to fraudulently aquire the documents necessary to get a DMV to issue you an ID for someone else. Etc.

If IDs didn't expire, none of this would matter because fakes would continue to be printed with issue dates of runs with less stringent security features, and fakes made before those features went into effect would be valid forever.