"Pay for their own electricity" is just not an actual thing that seems to be doable at this scale without those externalities. You're talking about sites that are, say, 800MW or 1GW or more... these use more power than tens of thousands of homes, and require entirely new power plants, interconnects, lines run, transformer costs, staff, etc. Typically, power companies amortize those costs and spread them over ratepayers, and when it's just part of the normal induced load of population growth, that's just how things go, and the system works.

In these cases, the rates that these DCs are paying for power are nowhere close to being able to fully absorb or offset the additional CapEx that the power companies are suddenly tasked with, even if they put up, say, the capital for IC, which is usually what's required. So the remaining new shortfalls get spread over the remaining ratepayers, ie, everyone else. If the demand wasn't induced by the build of these massive sites that, strictly speaking, aren't "necessary," then the rates wouldn't have to climb to accommodate them.

Frankly, there should be laws against power companies raising ratepayer rates to accommodate infrastructure investments driven solely by DC/fab load inducement.