If the absolute superlatives are superlatives, why can we use buenísimo? Shouldn't we use mejorísimo?
Bueno is just an adjective, it means "good". It does not compare things.
Buenísimo is not an absolute superlative but a normal one. It just means muy bueno → "very good". Many things can be buenísimo at the same time.
Mejor is a comparative. It is used to say that something is better, more bueno than something else: Toyota es mejor que Honda → "Toyota is better than Honda".
The particle -ísimo is a suffix that means "very": dificilísimo → "very difficult", rarísimo → "very rare". It is used to easily form superlatives from adjectives in Spanish.
It doesn't make sense to say ❌mejorísimo, just like it doesn't make sense to say "Toyota is very better than Honda".
Some adjectives like bueno use a different word like mejor when making comparisons, but in most cases you just use más: Tú eres más guapo que yo → "You are prettier than me"; El Empire State es más alto que el edificio Chrysler → "The Empire State building is taller than the Chrysler building"; Otras preguntas podrían ser más interesantes que esta → "Other questions might be more interesting than this one".
Finally, you have absolutes. In Spanish, you form those by preceding the comparative with a definite article (el, la, lo → the) — but, unlike in English, the form doesn't change. So: Tú eres el más guapo → "You are the prettiest"; El edificio Burj Khalifa es el más alto → "The Burj Khalifa building is the tallest"; Esta pregunta es la más interesante → "This question is the most interesting".
Mejorísimo would only be used in an exaggerated, almost wrong-on-purpose way, just like "very best"; and, just like it, it is grammatically incorrect (though "very best" is much more established and accepted in English, while most everybody would recognize mejorísimo as wrong in Spanish).
Note: if mejorísimo was right, I think it'd probably be spelled mejorcísimo instead, just like its diminutive mejorcito. Spanish 🤷♂️.