So everything above 63 wt% SiO$_2$ is just "felsic". But there are some rhyolites with more than 76 wt% SiO$_2$ (some of the 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 "felsic". That doesn't mean they are not different, just that we never bothered making a fifth field. But those fields are just arbitrary, we could have split the silica spectrum into twelve fields, or just two.
As mentioned in a comment to another answer, the terms "basic" and "acid" in relation to igneous rocks are outdated and stem from an incomplete understanding of rock chemistry in the past.
Better terms are "ultramafic", "mafic", "felsic", etc. As to your question - why aren't there "ultrafelsic" rocks? Let's put it into more quantitative numbers.
Felsic rocks (or acid, in the old terminology) are rocks with high SiO2 content, usually higher than 60% or 65%. Your hypothetical "ultrafelsic" rocks would then have maybe 85% or 90% SiO2.
The reason rocks like that form is because crystals with low SiO2 contents are removed from the melt, enriching the residual melt in SiO2 and push them to the "felsic" field.
At some point, the melt is sufficiently rich in SiO2 and quartz becomes a mineral that forms from the melt. Quartz is 100% SiO2, so it stops the melt from becoming richer in SiO2. If by some act of magic the melt reaches 90% SiO2, it simply crystallises more solid quartz, removing the excess SiO2 from the melt, and lowering it to more reasonable ~75%.