NNPC cabal and vested interest

Former CBN Governor, Sanusi Lamido accused NNPC of withholding and diverting Nigeria's oil revenue, running into billions of dollars during the administration of President Goodluck Jonathan and this got him sacked.

During Buhari's govt, former Petroleum Minister, Ibe Kachuckwu also accused NNPC of mismanaging Nigeria's oil revenue and this also got him sacked.

Now, NNPC is also being accused of the same allegations.

What exactly is wrong at NNPC and why are govt appointees who raised questions about issues at NNPC being sacked?

Is NNPC more bigger than Nigeria?

Who are the cabals and vested interest at NNPC that are wielding power in the country and coercing presidents to sack their ministers who raised critical questions on NNPC?