Because he can’t.
Putin has steadfastly referred to the invasion of Ukraine as a “special military operation”, even to the point of banning the words “war” and “invasion” from the state media. Many have wondered why he doesn't simply declare war and go to full mobilization.
I submit that it is because he can’t. To declare war is a formal act whereby one state declares war against another state. You don't declare war against your own country; you “put down a rebellion”, you offer to “protect the people”, or engage in “pacification”, or, in Putin’s case, have a “special military operation” for the "demilitarisation and denazification" of Ukraine.
To declare war is to publicly acknowledge that Ukraine is a sovereign state, which completely undercuts Putin’s assertion that "Ukraine never had a tradition of genuine statehood".