It's probably because with Altissimo, if the appropriate voicing for a particular Altissimo note is not prepared in advance before the note is hit then the Altissimo note gets missed.
Mixing up regular things with Altissimo notes can help.
Playing a scale run that starts on say middle Ab and then descends to low Ab and then jumps straight into Altissimo Ab is a good thing to do because the jump from low Ab to Altissimo Ab requires a quick precise voicing change and it is a good thing to practice.
Also, jumping from high Altissimo notes to lower Altissimo notes and then into regular range and back up to Altissimo range etc etc.
The jumps to Altissimo notes require quick precise voicing changes which is not the same as playing Altissimo notes chromatically or randomly at a leisurely speed where the voicing doesn't really need to vary as much for close together Altissimo notes as opposed to the voicing change needed for low regular range to Altissimo range notes for instance.
Once someone is playing a Altissimo F# then it's not that hard to then play a Altissimo G because the player is already in the Altissimo range with roughly the right voicing but jumps from lower Altissimo up to higher Altissimo or regular range to Altissimo range is another thing that needs practice to do and it also helps in developing precise Altissimo voicing control.
In other words, just mix Altissimo notes up as much as possible with other things and keep practicing it.
Where players that can't play Altissimo or can't play Altissimo very well fail seems to be that they don't realize that their voicing has to change and they mainly just have a fixed voicing instead of a variable one.
Anyone can miss Altissimo notes by simply not changing their voicing, even experienced Altissimo players if they want to.
If someone has a variable voicing approach to sax playing then they will eventually be able to play Altissimo notes whenever they want to.
The voicing is the whole oral system pre preparing for a note by involving muscle memory and feel.
Practice and a good ear are needed and persistence as well.
