There are scopes with built-in isolation or differential inputs, quite rare anyway.
Speaking of scopes with 20MHz to 200MHz bandwidth:
It's difficult to isolate DC to high bandwidth inputs from the main circuit ground, and the main circuit ground of an oscilloscope is earthed for safety reasons. So for most scopes the inputs aren't isolated for cost reasons.
There's a small selection of scopes available in this bandwidth range that feature isolated inputs and allow connection to dangerous potentials (up to some 100V), but these are quite a bit more expensive than the non-isolated ones.
Speaking of higher BW (500MHz to several GHz) - like most other RF stuff, there's no other way but to solidly bond input cable shield to the chassis / shield, and that shield can be isolated from earth for a portable device, or often is earthed for a stationary device. Anyway, it's hard to completely and safely isolate the shield from accidental touch by the user, so a non-earthed handheld device usually doesn't allow higher potentials than ELV (extra low voltage) on its terminals.