Letting Russia taste its own medicine
When the Russian army invaded Crimea and Donbass, RU government denied any involvement:
- "Их там нет" (they aren't there) - the famous phrase of V.Vladimir Putin when asked about the Russian soldiers' presence.
- "Форму можно купить в любом магазине" (anyone can buy Russian uniform) - from the same speech. The claim was that the Russian-speaking local people self-organized, captured arms from the Ukrainian army and rebelled against the Ukrainian government.
Later, Russia halfheartedlyhalf-heartedly admitted helping the "rebels".
Even later, this claim was silently, but completely dropped as proofs about the opposite fact piled up.
An important point in the Russian propaganda is that Russia fights NATO and Ukraine is only a tiny facade. The propaganda pictures countless NATO instructors, officers and even ordinary soldiers in Ukraine. This point has its supporters and beleivers in Russia, in Ukraine and even in NATO countries.
Another important point in the same narrative is that the "West" holds Ukraine hostage and Ukraine soldiers are fighting against their own will. This is the whole idea of "liberating" Ukraine.
On the other hand, not fueling Russian propaganda is considered important by everyone (except, of course, Russians).
In a sense, the "red lines" that the western partners of Ukraine are reluctant to cross, are based on the perceptions of the general Russian population.
The propaganda is not omnipotent - even in Russia.
This precludes sending any military personnel in Ukraine (if there is at all a compelling reason to do so).