iPhone 10W charger, 7Watt Hour battery - takes hours to charge?
There are at least three things you are not considering:

As Turbo pointed out in a comment, the phone is using some power while the battery is being charged.  Not all the power from the charger goes to charging the battery.

Batteries aren't 100% efficient.  More energy needs to be put into them when charging than what you get out when discharging.  The circuitry around the battery that manages the charging and discharging also has some losses.

Just because a battery is rated for 10 Wh doesn't literally mean that's a good way to charge it, even if it were 100% efficient.  Wh is a unit of energy, and doesn't imply charging time and rate.  You could just as well express the energy in Ws, but attempting to charge a cell phone battery in one second would cause pyrotechnics.

Depending on the battery, it's life may be extended by charging from empty to full over more time than one hour.  Note that it takes more than one hour to run down a fully charged battery, even with everything on.