There could be many reasons. I don’t know where they are deploying them, how much training the crews have had and many others that factor into survivability. I’m not saying the Abrams is the best tank ever, just saying that there are many factors that can contribute to why they aren’t doing well in the field for the Ukraine.
For the same reason they have sent billions of dollars there…to show their support. This isn’t the first country that the US has provided equipment for with little or no training…or without critical equipment onboard (i.e. specialized RADAR or electronics systems)