Public utilities can't do the same? Moreover if the implication is that large tech companies are somehow getting great prices at the expense of residential users, what does that mean for the electric generators on the other end of this transaction? Why are they leaving money on the table by selling to large tech companies for cheap?
These companies are regulated and can only charge for the costs they incur plus a flat profit on top of that of 10% or so.
The datacenters give allow them to justify building a lot more capacity to serve them. That increases costs, which means that 10% added for profit is now a bigger number and they can give bigger returns to their shareholders. But those profits are extracted from the existing customers who now see higher bills to cover the costs of expanding capacity to serve the datacenters.
It's a question of aligning incentives.
The effect has a name: the Averch–Johnson effect, named after the Harvey Averch and Leland Johnson paper: "Behavior of the Firm Under Regulatory Constraint"
Fun stuff: https://en.wikipedia.org/wiki/Averch%E2%80%93Johnson_effect