Because society uses labels to delineate the boundaries between different legal regimes and sources of authority, and adhering to those boundaries is important. The FCC (Federal Communications Commission) can't point to the traffic on a fiber cable within a data center and say it falls within their jurisdiction because that's also "communications."
It's especially important in this context, because the half-century-old laws and regulations designed to control soot emissions from factory smokestacks is ill-suited to managing CO2 emissions. The fundamental premise of the Clean Air Act is that pollution can be controlled at the source with control technologies (RACT, BACT, etc.).
This focus on control technologies puts well-defined guardrails on the EPA. The EPA can't go around making big decisions about how factories work or what people produce. They can simply tell you to put the best reasonably available scrubbers on your factory. That approach completely breaks down when it comes to CO2, because there is no such thing as a "CO2 scrubber." Treating CO2 as a pollutant puts the EPA into a role of supervising the energy sector that the agency simply was never designed to do.