Vocalic Equivalance

if a sound-formation is totally open then it is vowel then if we introduce a bit of constraint, it becomes an approximant consonant.

Vocalic equivalent between ɑ: and ʕ̞

The open back unrounded vowel (represented as [ɑ], similar to the "a" sound in "father") is produced by lowering the tongue and pulling the tongue root back toward the throat (pharynx). If you narrow that pharyngeal space just a fraction more, the vowel transitions into a pharyngeal approximant [ʕ̞].
So, phonetically, [ʕ̞] has just enough narrowing in the pharynx to be classed as a consonant, whereas [ɑ] keeps the pharynx open enough to remain a pure vowel.
When linguists say that [ɑ] is the vocalic equivalent of [ʕ̞], they mean these two sounds share the exact same active articulators—namely, the tongue root backing toward the pharyngeal wall—just set at slightly different distances.
Vocalic Equivalance

But why not call all approximants as vowel?

Vowels are produced with an entirely open vocal tract. Airflow is completely laminar (smooth), with no local friction or turbulence created in the mouth or throat.
Approximants bring two articulators (like the tongue root and the back of the throat) close enough together that they reach the threshold of critical constriction. If you narrow the space just a hair more, you get audible friction (a fricative).

Articulation Site 📍	Vowel (Syllable Core) 🎵	Approximant (Syllable Edge) 🛑
Palatal (Front/Hard Palate)	[i] (like in see)	[j] (like 'y' in yes)
Labial-Velar (Lips/Soft Palate)	[u] (like in too)	[w] (like 'w' in win)
Pharyngeal (Throat)	[ɑ] (like in father)	[ʕ̞] (pharyngeal approximant)