Some regions, specially the EU at this point, have introduced new legislation to reduce waste (including batteries). Also, damage from leaking batteries is unfortunately an issue. People forget to remove batteries when the pedals are t being used, and even changing batteries when a pedal is mounted to a pedalboard can be a hassle. Using a power supply avoids all of this.
You might think it’s dumb, but that’s just because you don’t understand the bigger picture.
Including an IEC power supply into a pedal will cause the average pedals to almost double in size to accommodate the IEC inlet, the power supply itself, and all the necessary clearances/creepage distances for safety. Then there’s the issue of dealing with all the bulk from the IEC corsets.
Then, each pedal would need to be safety and EMC certified, which adds tens of thousands of dollars to the cost. A pedal that operates at 9-18V is exempt from (most) safety regulations, and if it doesn’t have a processor or boost converter then it’s also exempt from (most) EMC regulations.
External power supplies are safety and EMC certified, the costs spread out over tens or hundreds of thousands of units since they are used for so many different applications.
Since you are new to pedalboards, yes you are misunderstanding things.
Most pedalboard users choose to use a pedalboard power supply that has outlets that can be switched or selected based on a pedal’s needs.
This is most common, and can efficient way to manage power. There are a variety of high quality, multiple output supplies on the market. Some pedalboards even come with one built in.