There are so many uses for each and each has their place.
FPGAs are great for glue-logic like busses between chips and such. They are also quite useful for fixed emulation of other ICs no longer available. They can be used as a cheap solution to making ASICs. That is just the tip of the iceberg and I expect other users will have a lot more uses that I.
I teach online and I have students who can afford my services and other students who I help as charity, for the latter I send them a Raspberry PI to use as their own computer. Which is one of the big distinctions of a Pi as compared to an Arduino. That is, a Pi is a fully programmable computer with a von Neumann architecture, whereas Arduinos (or the vast majority of them) are Harvard architecture.
That basically comes down to memory. A Pi can load program code into memory and execute it where an Arduino cannot access new code nor modify its code at runtime.
This is a good topic. I look forward to what other users more experience have to say.