Why aren't there ultra acid igneous rocks?
Warning: not an igneous petrologist here, I'm more interested at what happens to magmas once they reach the surface than all the processes they might have encountered along the way...
That being said, I think it has to do with the source of magmas, that is the mantle, which is itself mafic. Hence partial melting of the mantle generates mafic magmas. These magmas can then differentiate into more &quot;evolved&quot; (i.e. intermediate or felsic) ones. But there are limitations to these differentiation processes. With fractional crystallization you make mafic minerals, pumping Mg and Fe from the melt and leaving it silica-enriched, but you will never reach a 100 % silica melt (after all mafic minerals are still silicates, they need some Si in their structure). Same with crustal assimilation: the continental crust is only andesitic (intermediate) on average (Rudnick 1995), and even with assimilation of more felsic (i.e. granitic) portions you would never reach a 100 % silica melt. The starting material is too mafic.

On a more &quot;philosophical&quot; note, I would add that despite the absence of the term &quot;ultrafelsic&quot; in the literature, one could still consider some rocks as being ultrafelsic. If you look at the chemical classification of volcanic rocks (Le Maître et al. 2002), the four fields are separated like this (silica content in wt%):

Ultramafic: SiO$_2$ &lt; 45
Mafic: 45 &lt; SiO$_2$ &lt; 52
Intermediate: 52 &lt; SiO$_2$ &lt; 63
Felsic: SiO$_2$ &gt; 63

So everything above 63 wt% SiO$_2$ is just &quot;felsic&quot;. But there are some rhyolites with more than 76 wt% SiO$_2$ (some of Mono Domes for instance, see Table 2 in Bray et al. 2017)! So there is no distinction between a low-silica dacite and a high-silica rhyolite: they all are just &quot;felsic&quot;. That doesn't mean they are not different, just that we never bothered making a fifth filed. But those fields are just arbitrary, we could have split the silica spectrum into twelve fields, or just two.