Why can't everything be FPGAs?

Then you could have programs dynamically create hardware depending on their needs.

Not everything is amenable to being partitioned into a fixed array of gates. If you're doing something that involves a lot of data transformations (sorry, the speed of light is finite so mutable data will be with us forever) you can do much much worse than random access memory.

Maybe one day we'll have hardware that is plastic enough that you will be able to transform it on microsecond time scales, but we aren't there yet.