The thing is that the US and NATO consider themselves to be these invincible armies, with the best training, the uber equipment and tanks, the best technology.
About Russia they only ever talk in disparaging terms (unless they want to justify raising defence budgets or funding an expensive weapons program). Russia they give credit for advances only in the underhanded and rogue tactics - cyberwarfare, propaganda news agencies, buying off Western politicians and the like. Yes when it comes to fighting in the open Russia would surely fall apart as a vampire exposed to sunlight - I mean nothing can stand up to those US Carrier Groups, EU Battle groups, NATO training & discipline, the F-22 Raptor, Leo 2 MBT, A-10 Warthog and all the rest of it.
So one can imagine their shock when in straight up warfare small numbers of Russian SF and artillery lighted up Ukrainian armored columns to the last vehicle w/o much difficulty, and Russian-trained DNR/LNR tankists smashed the advancing Ukrainian columns.
I mean of course the Ukrainian Army is also an inferior eastern one, with inferior eastern tech and Soviet training. But still this should not have happened - it had Western advisers training them, helping plan their ops. It had those NATO counter-battery radars.
So naturally these analystd gravitate to the only rational explanation they can find - that Russia intervened with its horde of conscripts and rusty old equipment and turned the tide by sheer numbers; naturally though at a huge cost that only said Asiatic despot societies can bear.