To be fair, they had no reason to reverse it because both Russia and Ukraine were part of the same country at the time. And after the fall of the Union, Russia had much bigger problems to face than to try and get Crimea back. They got a lease on the important parts of the peninsula so even after the breakup, there was no reason to take it back as the country was still in their sphere of influence.

That changed, however, when Ukraine started to pivot more towards the West. Which Russia, for its own strategic and geo-politicial interests, could not allow. This is what many people today forget. Self-Determination, Morals, Values, Agreements and all that do NOT matter in the slightest when it comes to Strategic Interests and Real Politik. As much as I don't support the invasion of Ukraine by Russia, I at least understand that it's not about "Putin having a small dick and wanting to expand his ego and power". Which is just an uninformed and overly-Emotional opinion to have and is only propagated by Western leaders because it's the only way to justify helping the Ukrainians to their own citizens.

But good luck finding any support for a nuanced discussion about this conflict. Any objective and nuanced look at this war includes the fact that Russia has to do this for its own security. They would rather oblitorate Ukraine than to have it join NATO or the EU and the more we in the West to bring Ukraine into the fold, the more aggressive the Russians will be towards them and us, jeopardising international stability and undermining the Balance of Powers politics that currently applies to much of the world.