If wind/solar is, truly, cheaper then other forms of electricity, then why are giant data centers not being built with wind/solar power ?

well, clearly, one can think of reasons, but there are large rural areas in the US that have a LOT of wind (the high plains) or a LOT of solar (desert SW)

Texas apparently has both

https://www.climatecentral.org/report/solar-and-wind-power-2024