Why don't they construct giant magnifying lenses to create heat and power?

We already do

This is probably what you pictured in your head, but it is exactly the same thing on a very large scale.
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Creating and storing power was the easy part. Far trickier was finding a way to deal with the stresses created by “radiation fluxes”

I see some logistical issues:

The lens would have to be HUGE to make a difference – Glass would be very heavy, and I am not sure plastic coule bear the heat.

Also, it seems this would need to be mounted well above ground to have the greatest likelihood of catching the largest amount of sun. How would this be built, and more to the point, how could it swivel and follow the sun’s path?

These people are doing something similar but are aiming the sun’s reflected rays instead of using lenses.

@Forever_Free Considering a small magnifying glass can start a fire very quickly I don’t see that it has to be so huge. One simply builds alot of them like solar panels.

Apparently, current solar power generators are more efficient.

A small magnifying glass can burn a twig or some straw. Or a piece of newsprint. Making enough heat to generate power is a bit more of an effort.