Why can't we produce cheaper desalinization plants?

It takes a large amount of energy to remove salt from water. Energy is pretty darned expensive. Also salt water is quite corrosive and unfortunately materials that are corrosion resistant are relatively expensive. The result is expensive desalination.

The obvious solutions are to: 1) reduce the energy cost - solar desalination is the clear winner since sun light is free; 2) design the plant with care to minimize the need for expensive materials.

The first answer to your question got it wrong on energy requirements. Flash evaporators can run on just three kilowatts.

There is an awful lot of heat just wasted. Look at power stations they use cooling towers to get rid of heat. That could drive a really viable desalination plant at very little cost and thus actually provide the power station operators with a profit!

There are many manufacturing processes that produce waste heat and all of that could be harnessed to run evaporators at a profit.

Nuclear waste is processed into glass bilts that produce 3 kilowatts of energy for fifty years and then only degrade slightly. This heat source could be safely encapsulated to provide the heat to run evaporators at virtually no running cost for well over a hundred years. The rabid anti nuclear lobby would no doubt kill this idea without even thinking about it.

There are many ways to de salinate water and many ways to power them cheaply. All it takes is the will and the initial build cost.

We are..

New technology is coming on board at this very moment

which drops the cost by 75% of older desalinization plants.

because we cant produce them is they need the demand for greater than supply for fresh water

A most intelligent question. I wish I had an answer for you, but I don't. My guess is they have not yet found a way to make enough of a profit from it.

Because we have to go to Mars!