1) They could not operate them. It isn't just about authorization sequence, it's about having all of the required electronics. You need satellites that point and guide the ICBMs. All of those were in Moscow hands. Even if Ukraine could ignite them, it could not launch them or set their paths, etc.
2) They did not have the budget to guard them, let alone maintain them, even less reverse engineer. The biggest risk was that rough states with deep pockets would buy those rockets on the black market (and Ukraine notably sold out most of their soviet arsenal).
3) Thus, the only real asset was the nuclear material itself. An asset that was more likely going to end up on the black market than do anything useful for Ukraine's defense.
