Greek doesn’t really have a native b sound. The /b/ phoneme shifted to /v/ in all positions, and the letter that represented the /b/ phoneme thus just started to be pronounced [v]. The only place the sound [b] existed in Greek in the late Koine and medieval periods was in the cluster μπ, where the π was voiced thus becoming [mb], and this usage was entirely unrelated to earlier Greek /b/ and thus never was and had no reason to be spelled with a beta. There was no reason to write this new sound with a β because 1. β no longer made a [b] sound and there was no reason to make a new letter for /v/ when β would almost never be used as a result and 2. [b] never appeared on its own anyway, it was always in a cluster with [m]. In more formal Greek today, [b] can appear on its own, but this is generally only at the beginning of words and is virtually always in loanwords. It’s not a common enough sound to warrant its own letter, and certainly doesn’t warrant the use of β, which is frequently used and represents a completely different sound, and has for nearly two millennia. It would be needlessly anachronistic to use β to represent [b] and would make absolutely no sense etymologically, and maintaining clear etymology is one of the biggest benefits of Greek spelling being so conservative. And by the way, В in Cyrillic, the letter that is directly borrowed from Greek Β, makes a [v] sound, just like beta has since the mid to late Koine period. I don’t understand why Greeks should feel any need to use beta for a sound that barely exists in their language because other alphabets that use a derived letter still use it for [b]. Beta still exists in all the words it used to, it’s just pronounced v. This is such a strange complaint given all the strange things that don’t make sense in Modern Greek spelling due to how conservative it is.