I guess you never use these words. They all used to be separate words until people started using them as compound words. It’s not like there is some government committee that decided to make them compound words. People just started doing it and it caught on.

Email
Username
Notebook
Backyard
Everybody
Anyone
Someone
Everyday
Cupboard
Playground
Railroad
Into
Cannot
Breakup
Setup
Takeover
Altogether
Already
Forever
Sometimes
Whenever
Overtime
Underground
Firefighter
Airport
Footprint
Crosswalk
Skyscraper
Underwater
Handshake
Landmark
Housekeeper
Bookstore
Grandparent
Hairbrush
Moonlight
Sunshine
Staircase
Waterfall
Wheelchair
Snowflake
Thunderstorm
Highway
Earthquake
Nighttime
Afterward
Birthplace
Classroom
Deadline
Dishwasher
Doorbell
Earthbound
Fingertip
Grasshopper
Handshake
Headphone
Hometown
Lifespan
Notebook
Overload
Pancake
Passport
Quickstep
Raincoat
Schoolhouse
Showroom
Starfish
Teacup
Uplift
Waterway
Backpack
Battlefield
Blackboard
Blueprint
Bookcase
Brainstorm
Breakfast
Bulldog
Carport
Clockwise
Daydream
Driftwood
Fireplace
Fishtank
Flowerpot
Footstep
Goldmine
Greenhouse
Guideline
Handshake
Heartbreak
Horseback
Jellyfish
Keyboard
Lighthouse
Nightfall
Overboard
Playhouse
Quicksand
Rattlesnake
Roadmap
Seashell
Sidewalk
Spotlight
Springtime
Stockpile
Tailspin
Taskmaster
Textbook
Timeline
Touchdown
Tugboat
Underdog
Workbench
Windswept
Workforce

Ok, I’m done.