Once again, the OP makes comments about the process of making music, in this case improvisation and practice, that are incorrect or only partially correct. I am not "on a mission" to prove him wrong as he has said previously, I simply think that it is important to set the record straight about these issues since this is a music thread after all. The comments are made with a sense of authority and with the apparent backing of musicians. In this most recent case the problem is that the comments are tantamount to saying something like "global warming is real because Einstein determined that E=MC2". Huh? Sorry, O-10, but you don’t have it correct. I encourage you try and understand the process better and would be glad to help you with that. Several comments have been made already by other posters that are good and partial explanations. In the meantime, jzzmusician’s comment about practicing improvisation is spot on and explains very well why improvisation can be and must be practiced IN ORDER TO ALLOW the musician to be spontaneous when he is actually performing. Improvisation CAN be "akin to conversation" (Bill Evans and Scott LaFaro pioneered the "conversational" style of improvisation with their classic trios; but, that is a particular style of improvisation and improvisation is usually akin to a monologue with the support of the other musicians and certainly not "dependent" on what came before it. Again, it CAN be influenced by what came before it, but certainly not "dependent" on it.
O-10, in some respects you are on the right track re "the process" and its always a positive thing to actually think about these things. The problem here is the incompleteness and absolutism with how you view these topics. As someone very wise once said: a little information is a dangerous thing.
https://m.youtube.com/watch?v=t-vItf0G05MTHAT was Herbie’s comment. A great example of a musician (Miles) with fantastic musical instincts and ears; and SUPPORT of the spontaneous process. I suppose one could say there is something "akin to conversation" in that example, but it is, more than anything, an expression of Miles’ overall musical attitude of "go with the moment" and emblematic of his attitude for always accepting and reaching something new. Herbie played a wrong chord, had Miles not previously "practiced" playing over that type of (wrong) chord, that moment would not have been possible. That knowledge became part of his arsenal which he could draw upon in order to be spontaneous.