According to Frellesvig, B. (2010). A history of the Japanese language. Cambridge: Cambridge University Press, in Late Middle Japanese the verbs did have the volitional forms that is implied by the addition of -う to the 未然形 form; that is, せう(=syoo), 開けう(=akyoo), 寝【ね】う(=nyoo), 起きう(=okyuu), 見【み】う(=myuu) in parallel with 書かう(=kakɔɔ) and 持たう(=motɔɔ).
However, in Modern Japanese, the volitional form have changed; this is best explained as an analogical change.
- As /ɔɔ/ (=あう) and /oo/ (=おう) merged, the suffix was reanalyzed as -(y)oo, making the volitional form of 上一段活用 [見【み】う(=myuu)] and 上二段活用 [起きう(=okyuu)] outliers to the paradigm
- As the former 二段活用 became levelled to 一段活用, the conjugation paradigm of 開け- was no longer alternating between ake- and aku-; instead it was levelled to a single form, ake-. The volitional form akyoo is thus an outlier to the paradigm
Hence we have 開けよう, 寝よう, 起きよう, 見よう as the new paradigm of volitional forms, resolving both of these outliers while also making -う/-よう parallel with -せる/-させる, -れる/-られる in that they care whether the preceding verb is 四段 (or, 五段 after the spelling reform) or not.
Languages tend to make changes unpredictable from sound laws, and in many cases (especially in verbal paradigms) such changes are fuelled by analogical levelling. For instance, Old French had tons of vowel alternations (caused by sound laws from Latin to Old French) such as leve / laver, aime / amer, trueve / truver, many of which have lost the alternations in the contemporary language (lave / laver, aime / aimer, trouve / trouver).
https://en.wikipedia.org/wiki/Old_French#Verb_alternations
