Because they want to cheat, or they feel it will help "one side" more than the other.
This is why the arguments against always fall flat, because the so-called obstacles to register and/or vote are easily debunked.
Okay. But there are voter ID laws in already in place. Voter ID laws in the United States require a person to provide some form of official identification before they are permitted to register to vote, receive a ballot for an election, or to actually vote in elections in the United States. The Help America Vote Act of 2002 requires voter ID for all new voters in federal elections who registered by mail and who did not provide a driver's license number or the last four digits of a Social Security number that was matched against government records.
Real ID is becoming the norm in the U.S. Real ID requires you provide proof of citizenship. Beginning in 2025, folks cannot board a commercial flight in the U.S. without real ID. Presently thirty-six states have laws requiring voters to show some form of identification at the polls. Thirteen states do not require voter ID to vote in person with some exceptions. Most require you sign a form (called an affidavit) and vote. Of the states these states, it appears that Hawaii has the least requirements to vote in person. Eight states and Washington, D.C., allow all elections to be conducted entirely by mail: California, Colorado, Hawaii, Nevada, Oregon, Utah, Vermont and Washington state.