They rust on solid if not removed every 5 years or so , cleaned & greased.
My method is to put a pair of scissor jacks between the wheel & the frame to put a moderate load on the wheel.
Then hit the axel with an air hammer till it moves a little forward and the jacks go a bit loose.
Tighten the jacks and repeat.
Can take a day or two
On really tight ones I get something in on the axel to clean it, wire brush, wet & dry etc then hit the inside of the hub that is clear of the axel with a cup brush to clean it.
Apply some penetrating fluid, tap the wheel back on then repeat the process at infinitum.
Do not use excessive force as the axel is held in with a clip which can come loose then you have to pull the box apart.
Or pull the box apart & press the axels out.
You can heat the wheel dull red and pull it off but repainting takes a lot longer.