Why do XeO and XeF8 not exist?
$\ce{XeF8}$ is not known to exist though O.N is +8. Why is this so? 


At least 2 compounds have been reported that contain the $\ce{XeF8^{2-}}$ unit. See, for example: 


$\ce{(NO^+)2[XeF8]^{2-}}$ (reference)
Metal salts of the form $\ce{(M^{+})_2[XeF8]^{2-}}$ where M is a metal salt such as $\ce{Cs, Rb}$ (see the above reference) or $\ce{Na}$ (see p. 62 in Advances in Inorganic Chemistry, Volume 46, A. G. Sykes editor; link 


The $\ce{XeF8}$ portion of the molecules approximates a square-antiprism geometry. 

     

(image source)


  the lowest oxide of xenon $\ce{XeO}$ is not known though O.N is +2. Why?


Perhaps the molecule is unstable, favoring disproportionation to other xenon oxides plus oxygen. For example, Andreas Hermann and Peter Schwerdtfeger suggest the following pathway at high pressure:

$\ce{3XeO → Xe3O2 + 1/2O2}$     

The authors go on to note "we do not find $\ce{XeO}$ to be stable at any pressure" (link to full paper).

Edit: See orthocresol's comment below. He argues that at ambient pressure the following decomposition pathway might be more likely:

$\ce{XeO -&gt; Xe + 1/2 O2}$