Why An oscilloscope displays a signal as a plot of voltage versus time on a screen. To do that it needs a vertical channel amplifier to scale the input signal's amplitude and a horizontal channel that sweeps the trace across the screen (or amplifies a second input in X-Y mode). No other common bench instrument uses that two-axis deflection architecture.
Watch out A signal generator produces test signals rather than displaying them, and ohmmeters and ammeters are single-value measuring instruments with no display axes at all.
Horizontal + vertical = a screen with two axes, so it's the scope.