It was created in response to the September 11th attacks. Before then, for constitutional reasons of Separation of Powers, most IDs in the US were issued by the states and they had varying standards of what was on the IDs, what criteria was required, and how they were issued. Some were easier to create fake versions of (mostly used by college students to buy alcohol), others were easier to obtain authentic IDs using fraudulent information. IIRC some of the hijackers used authentic IDs obtained fraudulently when they boarded their flights.

The Real ID Act of 2005 tried to standardize IDs nationally and the criteria for issuance, including verification of documents. It’s taken 20 years to get all of the states on-board and compliant, so they only started requiring passengers to present a Real ID recently, but there are a lot of people out there who haven’t obtained one yet based on when their state started issuing them and when they’ve had to renew their driver’s license. I think my state has offered Real IDs for a decade or more, but the Real ID was more expensive and optional for a while. I didn’t bother getting one until a couple years ago, partly because I already have a passport and that’s also a valid ID for flying.