Because more people will complain about no sound than too loud sound.
100% probably means "as stored or received" l. Most sounds provided will be adjusted to be somewhat inside the ceiling before you get distortion. But perceived volume is dependent on many things like frequency. And audio compression, where volume peaks are lowered to allow the whole thing to be made louder is often used for ads, so they sound much louder.
"almost all" probably counts a lot of phones, tablets, and "devices" with minuscule, minimalist speakers that cannot produce non-distorted sound at any loudness no matter what the volume setting is.
most people don't care. they may or may not be able to perceive a difference. they want a "hear better" knob and this is what they have so they assume that's what it is.
The mere fact that an application provides you with a volume control suggests the idea of regulating volume with it. If the intention was to always use the headphones/loudspeakers' own control, why include a volume control at all?
100% probably means "as stored or received" l. Most sounds provided will be adjusted to be somewhat inside the ceiling before you get distortion. But perceived volume is dependent on many things like frequency. And audio compression, where volume peaks are lowered to allow the whole thing to be made louder is often used for ads, so they sound much louder.