Well, concrete is solid and water is liquid.

I know it sounds obvious, but have you ever seen in your life a plastic flask or tank or jerry can that after several years leaked?

It is of course doable, but how durable will it be?

Imagine the kind of vibrations and shocks a jerry can used as a counterbalancing device in a relatively hot environment needs to bear.

A good, environment-friendly idea would be to make the concrete counterbalances a "standard" (in size and holes/bolting) and re-use them.

Nowadays in most countries (excluded the developing ones) the market for washing machine is almost exclusively a replacement one.

You get the new washing machine (without counterbalance) and you bolt on it the one from the old machine, and you save also the cost of the plastic water container.