Nitpick: These are names with no _vowel letters_; many (if not all, depending on how you define what a vowel is) of these letters are pronounced with a vowel.
maybe in English but not in Slavic languages (at least not in western Slavic).
"prst skrz krk" [ˈpr̩st] [ˈskr̩s] [ˈkr̩k]
That's a vowel in there.
In Latin & Ancient Greek, which are the origin of the grammatical terms that most people learn in school, only A, E, I, O and U could be used as vowels. Because of that, when most people hear about vowels, they think about these sounds.
Nevertheless, in most languages there are also other sounds that can be used as vowels. Even in English, words like "listen" or "often" have 2 syllables and the vowel of the second syllable is "N". Another example is "people", which also has 2 syllables and the second vowel is "L". Among the English speakers, there are variations. In words like the previous examples, some speakers pronounce a short central vowel before the final "N" or "L", which remain consonants, but other speakers transition directly into the final "N" or "L" from the previous plosive, in which case "N" or "L" are the vowels of the second syllable.
Like in English, there are many languages where the liquids (e.g. "L", "R") or the nasals (e.g. "N", "M") may be used as vowels. While English has restrictions for where such vowels may appear, other languages allow their use in much more kinds of sound environments.
These kinds of sounds can be used as vowels when alone or when surrounded by sounds of lower sonority than them, because when they are pronounced there are also paths for the continuous expiration of air through the mouth or nose, even if they are more constricted than in the sounds more frequently used as vowels, e.g. AEIOU (i.e. like you can pronounce continuously AAAAAAAA, you can also pronounce for as long as you want LLLLLLLL or NNNNNNNNNNNN).