In addition to what others have said, Russia’s fleet has been very susceptible to drone attacks. They have gone to great efforts to protect their ships from such attacks, and in doing so it has greatly reduced their ability to quickly move the ships. It’s likely the reason Ukraine has been able to guide cargo ships out of Odessa and into Romanian waters.

Holding the platforms will likely be easier than taking them, because Ukraine can set-up anti-air and anti-ship defenses on the platform. This weaponry probably couldn’t be set-up well on small ships, but the stable platforms should provide good launching points.

ETA: Russia also likely does not want to destroy the platforms, as they are both strategically useful and useful for economic purposes should Russia ever be able to operate them as oil rigs. So, they can’t simply blow them up without giving up something they want.