Why we never found the Missing Links between Homo Erectus [sic] and Homo Sapiens [sic] (that's us).
There was none, because there is no gap. H. erectus and H. sapiens are just chronospecies of each other. It’s kind of like asking for the missing link between teenagers and young adults.
The number of difference between the two species is estimated at a whopping 1,650,000.
What do you mean by “number of difference”? Base pairs? Mutations? Genes? Significantly different physiologically different traits?
That are about 4 (harmless) mutations per year!
That is not anywhere as implausible as you think. Every human child has on average 45 mutations compared to its parents (80% of them on the father’s side due to spermatogenesis being a continuous process whereas ova are arrested during embryogenesis). Now, that is less than four mutations a year, but by less than a magnitude. Also, keep in mind that these are just individuals when we are talking, when evolution is working on populations, that mutations are not limited to changing just one base pair at the time and that there has been a bottleneck early in early H. sapiens (more likely the unusually extensive second-most-recent glacial rather than the Toba eruption) leading to gene drift.