Because they have a proven track record of lying about EVERYTHING. They are essentially a dictatorship run by a madman, and they have always behaved very weirdly and sneakily.
They literally said yesterday that the West "started" the nuclear bomb sabre rattling when they are on fucking record as saying "we won't rule it out". They LITERALLY started that conversation with a passive aggressive threat and then claim they didn't do that.
Incredibly dishonest country. Don't trust a single word they say. Now, Ukraine are not flawless and in the months before this started the UK media openly referred to them as "Europe's most corrupt country" and made numerous references to the neo Nazi elements. Which exist in all Eastern European countries and Russia itself.
I don't buy into them being suddenly plucky heroes. But they are acting like plucky heroes since you know, Russia invaded their country.
They are potentially bad guys. But in this conflict they are not. By no means at all.