Why didn’t Erda just keep an extra primarch instead of LE2 (Leetu)

As the title says, it would’ve made sense for Amar Astarte to keep a LE2 type “specimen” and for Erda the “mother of the primarchs” to keep a secret primarch, if such a thing was even possible.

EDIT: puts on tinfoil hat

Given that LE2 already has 3/7th of “legion 2” in it, it could even have been the 2nd primarch, doesn’t need to be a 21st (or 22nd depending on how you look at it, primarch.