Right, lots to unpick here.
Firstly the tanks that Britain, America and Poland/Germany are supplying are not necessarily better tanks. The Leopard 2s and Abrams in question are not the latest models and to be honest are very dated. The British Challengers are the latest model but use different ammunition to the Leopards and Abrams due to having a rifled barrel. All 3 use different ammo to the Soviet models.
Secondly the numbers are tiny. 14 Chally 2s, 30 Abrams, 14 Polish Leopards (14 in the number of vehicles in a Company/SQN) So that's currently enough for 4 Squadrons but with 3 different types you have to have 3 separate supply chains. Tanks use a huge amount of spare parts and having to source spare parts for 3 different models is going to be a massive headache, also they all have there own ammunition requirements which are completely different to the current Ukrainian ones. The point about the American tanks you made above is actually a fair one. The Abrams has a Gas Turbine engine that is a real gas guzzler and most militaries cannot sustain it in the field long enough to make it worth while, only the Yanks have the resources to do it. I believe the export model has a more standard diesel engine or they wouldn't sell any.
Thirdly Western tanks are bigger, Heavier and require a loader (crew of 4). A chally 2 for instance is 70 odd tonnes, a Ukrainian T-64B is about 45 tonnes. All the Ukrainian army heavy transport, bridging equipment and supply chains are geared up for smaller tanks with smaller crew (Soviet tanks have an auto loader crew of 3). It will take about 6 weeks for a trained commander, driver and gunner to convert from a T-64 to a western model with the need to train loaders from scratch and change SOPs for the rest of the crew.
So we have lots of small numbers of bigger, different tanks which are not really any better than the upgraded Russian ones (T-90s is arguably better than all 3). This could be a logistical nightmare for the Ukrainians and could cause them more problems than it solves. Logistics win wars more than weapons
The ideal situation would have been for Poland to give the Ukrainians some of its PT-91s and T-72MIR which are both based on the T-72 but have been upgraded to a much better standard than the models of Abrams and Leopards on offer and the crews could almost jump in an drive off in them but Poland are not going to give these away as they could find themselves next in line for Russian aggression. Giving away a few aging Leopard 2 is one thing but its not going to gut the core of its most capable models no matter how much they want Ukraine to win.