Mbappe is of course exceptionally good. Real Madrid have a great striker in the person of Benzema. Maybe it even performs better than Mbappe. How would you actually get those 2 to play together? I think Benzema performs so well because he has a free role, when Ronaldo was there it wasn't. If Mbappe had gone to Real Madrid, it would presumably be at the expense of Benzema. 1 top striker is enough, and you don't want to put 1 of the 2 on the reserve bench.
If Real Madrid signed Mbappe he would have been converted to a winger using Mbappe-Benzema-Vinicius to spearhead their attack, of course any of the trio can score goal if the opportunity unveil itself, that combination would have the most potent and formidable attack in Europe. Alternatively if Benzema is injured Mbappe can step in or be drafted as their main striker, Benzema free role that culminated in his scoring of goals was due to chances created by their wingers like Vinicuis and midfielders e.g Modric, Camavinga e.t.c.