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If the rise time isn’t 0, it’s not a square wave.

And if that's your definition then there's no such thing as a real square wave.

Just like no physical objects can have a perfectly square corner, there will always be some radius, even if it's just 1 atom

The reason making a true square wave is hard is that there are physical properties of real life electrical components that prevent voltage (or current) from changing instantly. Similar to how we can't instantly accelerate a mass from 0 to some speed, it's physically impossible. The faster you try to do it, the harder it is due to inertia. In electronics, there's always natural capacitance and inductance slowing things down. If you want a 10v square wave, you have to push some amount of electrons through some amount of capacitance and inductance and that, while it can be very fast, is never instant.