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Most european countries* really don't have many spare main battle tanks, and didn't have the vast stockpile of de-commissioned near junk that the Soviets bequeathed to Russia.
Also it is a good opportunity to clear out stuff, (like the US M113 'armoured' personnel carriers), that have been superceded by modern infantry anti-armour weapons.[*Greece has more main battle tanks than any other EU country?]
Come on, Western leaders, do your bit - you're donating less to the Ukrainian war effort than Russia itself!