There's two moving parts here
Ukraine's GDP wasn't large before and has dropped considerably
Ukraine had an estimated $155B GDP (USD) in 2020 (for comparison, US GDP for 2020 was around $21,000B). They're estimating just under $120B for 2022, but I suspect that's high. With Russians occupying the Black Sea ports, exports of major things like wheat have become considerably more difficult, plus much of the country has been devastated by the war and a great deal of infrastructure has been destroyed. Ukraine didn't have a great economy before the war. Ukraine's victory is far from assured still, so their GDP will likely shrink even more the longer the war goes on. Without GDP, you can't exactly raise the money necessary to buy these advanced weapons. Even if they were at pre-war levels, that's not a very large economy to buy advanced weapons with.
Then there's the cost of weapons. One F-22 Raptor fighter jet costs about $150M. Ten of them would be $1.5B, or around 1% of 2020 GDP. For comparison, Germany just agreed to ramp their military up to 2% of GDP. And 10 jets wouldn't be a lot against the Russian air force. The same goes for the M1 Abrams tank. Let's round the cost to $10M per tank. 100 tanks would be $1B. Same problem.
Will other countries sell you their advanced tech?
The US, for instance, has sold the Ukrainians Javelin missiles, but also balked at facilitating Poland transferring their MiG fighters to Ukraine
The proposal Poland floated on Tuesday, however, would have involved the US more directly than the plan initially backed by Blinken and Thomas-Greenfield. Poland’s updated plan would have sent the MiG-29s to Ukraine via the US’s Ramstein Air Base in Germany, which also houses NATO’s Allied Air Command headquarters. Such a move could have more directly linked both parties to Ukraine’s war effort.
That added level of risk appears to have ultimately sunk the deal, though as Politico’s Alexander Ward and Joseph Gedeon point out, Poland could still unilaterally deliver the jets if it wishes to do so.
One of the keys here is "How much is too much for Russia?" There's a balancing act between sending them simpler arms and far more complex weapons of modern warfare (aircraft, armored vehicles, etc). Too little aid and Ukraine falls. Too much aid, and Russia starts World War III (which might be over in minutes if they launch nuclear missiles).
You also have to consider that you're also sharing your tech with a place where it could fall into enemy hands. Obviously the US is only selling marginal stuff the Russians know about. So selling F-22 Raptors isn't in the interest of the US either, since it's possible some Russian intelligence mole gets a hold of it.