True, but when it comes to actually using that rule-making power the agencies are given a lot of independence and are guided by the Executive’s policy goals (because the Executive appoints the various Secretaries and Directors and Commissioners). Agencies promulgate (and then enforce) a lot of rules and Congress would be bogged down having to pass laws for every single one. It’s a matter of convenience and efficiency.
Its does now, and has since the Clinton administration, but Congress didn’t explicitly have it before. Prior to the days of Gingrich and the “Contract with America”, disputes over agency regulations were worked out through negotiations or the courts and/or through laws promulgated by Congress that effectively nullified the rule. That process took time and effort and required having enough grown-ups on both sides of the aisle. In the 1990s the old-line GOP was replaced by a group that actively encouraged childish “I want it now-Now-NOW” behaviour (the link to their “free” market extremism not co-incidental), so the CRA emerged.
The CRA was thus an outcome of the increased political partisanship, divisiveness and obstructionism that’s become the norm since the 1990s. CRA was supposed to streamline the challenge process described above and was intended to stymie Clinton’s (and later Obama’s) progressive policies as expressed through agency regulations. Republicans can’t be thrilled that Dems have finally seen fit to use it and expose the GOP’s anti-consumer approach to something ordinary people use every day.