Why? It's not a native Windows on ARM app. Fornite uses kernel level anti-cheat. It's not a surprise it's broken. It'd be broken on x86 Linux under Proton or macOS with Whiskey too.
Linux on ARM at this point is in a more mature state than Windows with most distros having a huge percentage of packages available for ARM.
Right, "struggles" implies bad performance when in reality it simply doesn't work. Windows' translation layer handles user code, but not kernel drivers.
"No", Epic spent a lot of money on the mobile gaming market that never came. Its why Epic being booted from the Google and Apple stores was such a huge deal: torpedoed their dreams of monetizing loot crates and other degenerate scams in emerging markets.
Can't tell what build they used in the benchmarks, though.
Fortnite is not opensource, so the code is running through an emulator. Apparently, the quality of the emulators on Windows is severly lacking in comparison to what Apple has done.
