The problem why Western tank were not currently in Ukraine is because of logistic and support issue. Currently there are no US or NATO defence contractor in Kyiv providing support to fix these western equipment Ukraine were not equip to support when it breakdown, so say if a HIMARS or M270 MLRS engine has broken down, they have to tow it from wherever they are to Poland for them to fix it, this is not really economical. Which is why NATO has been sending unmodified ex-Soviet stock, Ukraine can fix them on the ground.
And the reason why there aren't US or NATO contractor in Ukraine is because it wasn't stable, but that was a few months ago, they probably need to look at places like Lviv and Kyiv or even further east near Sumy or Kharkiv are more or less stable enough to operate a maintenance center