Jitter specification of VCXO

In the jitter specification of VCXOs , manufacturers usually specify jitter from 12KHZ to to few hundreds of MHZ.
Why they do not specify it in close in?

I don't know the data sheet you are reading.
But, the VCXO is formally a free running oscillator in the sense that the frequency permitted may vary vs Vcontrol in a small range dominated by the Quartz.
For free running oscillator, that it'll be used into a PLL circuit, a figure of merit is the phase noise (relatively) far from the carrier.
(I speach about noise...it is close related to jitter).
Why this? Because, once the noise "far" is the lowest possible, the noise "near" the carrier is dominated by the PLL circuits and by the external reference.
Perhaps the manufacter think that 12 KHz is the lower boundry for the PLL-low-pass-filter designed around the oscillator.