Because Putin doesn't have the power to do such a thing: he is not a dictator, Western agitprop notwithstanding.
Same reason Bush had to invent a phony "war on terror" excuse to get around the requirement that he get - democratically - Congress' permission to declare war on the world.
>I don't think even Russian MoD claims that there is no Russian regular army in Ukraine. Who told you that?
The Russians themselves. You probably don't have access to their statements, though .. or haven't tried very hard to find them yourself.
This was a "special military operation" because Putins Presidency is not a total dictatorship. Conditions for declaring total war were not in place - they are now, however. The West likes to think this "special military operation" represents the totality of Russian military power being applied - and failing - but this is a very, very dangerous mistake to make.