Nato wants to be united. If they attack Russian forces in the Ukraine, Russia will likely launch hypersonic and ballistic missiles at the airbases Nato planes are flying from - in Poland, Germany, Romania, US carriers in the Med. Many of those countries are likely unwilling to be bombed by Russia. Some, like Hungary or especially Bulgaria, have not sent any aid to Ukraine at all- and are virtually Russian allies.
For the record, US airbases in Iraq have now been bombed twice by Iranian ballistic missiles - once under Trump, and once under Biden - without a response or retaliation. Just because we are willing to take it, doesn't mean our frontline allies are, and we are insisting on being united - for now.
There is also a lot more we could be doing first. Providing artillery, long range rockets, Migs, anti-air systems, tanks, long range drones - that all needs to be done first, like right now. And I certainly favor using stealth aircraft and Urainian-marked drones to conduct deniable airstrikes on Russian artillery, and shoot down their aircraft.