Because people don't all as sing falsetto
But if people just sang the falsetto bit then there would be no problem with having to learn the words
The Rollers were not known for their lyrical abilities:
"Remember"-"Shimmy shammy shom"
"Shang A Lang" "Doo whop be dooby do way"
"Summerlove Sensation" Doo wah be dooby doody"
Compared to the above "All of me loves all of you" is their version of "The Windmills of Your Mind".