Why is an oscilloscope's Attenuation switch placed on the probe?
The reason for putting an attenutator at the probe is to reduce the loading presented by the probe to the circuit under test. For a high-impedance probe (most probes) the cable looks like a capacitor, so it doesn't present much load at low frequencies but as the frequency goes up the loading can be significant.

The only real reason for using a probe in x1 mode is if your signals are very small and the scopes input noise is becoming a problem.

Most high end probes don't bother with the switch at all and are just permanently in x10 mode.