For some reason w is used instead of ʋ͡+ɰ or ɰʷ I don't know why
Sometimes that is because the two sounds contrast, as in Bunama. Generally, the secondary-articulation diacritics like ʷ are not deployed unless there is a significant phonological reason to do so, for example a general process of palatalization or labialization where writing a unitary segment that combines the phonetic properties of a sound would obscure the unity of labialization. There is a quasi-contrast in Mah Meri between [ɰʷ] and [w] between [ɰʷɔh] 'branch' and [wɔʔ] 'not yet', where [ɰʷ] derives allophonically from /ɰ/ before a round vowel. The consonant ɰ itself is very rare. In Phoible it is claimed to exist in <avea, but the source article does not use that symbol anywhere that I can see. It is attributed to a number of languages of Cameroon and Nigeria, where it is historically and phonologically a "differently-produced" velar fricative /γ/, which has a bit more friction (therefore stricture) that one finds in the classical glides [j w]. Your analysis seems to be based on the term "approximant" and an implied unity, but clearly "approximant" is not the same as "glide" (not an IPA term). The reasons for not using digraph-with-tie writing as in ʋ͡ɰ are similar – people rarely use the tie symbol. As a general rule, people do not go to complicated extremes unless there is no simpler choice.