His helo’s and jets have been getting shot out of the sky by stingers. Simply put, it appears US technology > Russian technology. Putin has to think long and hard about how many assets he wants to potentially give up.
Additionally, the pilots seem ill-trained. A few reports have 1 Ukrainian fighter pilot knocking down 6 Russian fighters.
In short, Russia has a lot of nukes. After that, good missile capabilities and a fairly well equipped navy. Problem is, their economy is too weak for the continued funding, maintenance, and training to keep the war machine “well oiled”.