Jump in the shower. Nice hot water. Then someone jumps in the bathroom shower off the adjoining bedroom, just on the other side of the wall, cranks up the hot water to max and... yours changes, typically not for the better. Only so much hot water is available and stored in the hot water heater. How about a third person in a third shower....Start running the dishwasher and clothes washer in addition to three showers and... and... and.... well, it just doesn't work out very well.
It's not any harder than that. There are solutions: two or three water heaters, lower flow shower heads, blah, blah, blah. But that's as hard as it gets.