Why do oscilloscopes use SMPSes instead of linear power supplies?

If proper engineering is followed, the switching noise shouldn't impact the readings and the oscilloscope weighs less, takes up less space, and draws less power. Thus, many benefits to a single increase in engineering difficulty.

The SMPS advantages over linear supplies, of compactness, high efficiency so low heating, light weight and wide input voltage range, are very valuable in portable equipment like an oscilloscope.

The principle disadvantages of SMPS over linear supplies are switching noise on the output and radiated EMI.

(Circuit complexity used to be considered another but not nowadays.)

Well-designed output filtering can reliably reduce ('remove') this noise and an oscilloscope carries a high enough cost to afford it.