Why on earth are o-scopes earth referenced?
Oscilloscopes usually require significant power and are physically big.  Having a chassis that size, which would include exposed ground on the BNC connectors and the probe ground clips, floating would be dangerous.

If you have to look at waveforms in wall-powered equipment, it is generally much better to put the isolation transformer on that equipment instead of on the scope.  Once the scope is connected, it provides a ground reference to that part of the circuit so other parts could then be at high ground-referenced voltages, which could be dangerous.  However, you'll likely be more careful not to touch parts of the unit under test than the scope.

Scopes can also have other paths to ground that are easy to forget.  For example, the scope on my bench usually has a permanent RS-232 connection to my computer.  It would be easy to float the scope but forget about such things.  The scope would actually not be floating.  At best a fuse would pop when it is first connected to a wall powered unit under test in the wrong place.

Manufacturers could isolate the scope easily enough, but that probably opens them to liability problems.  In general, bench equipment is not isolated but hand-held equipment is.  If you really need to make isolated measurements often, you can get battery operated handheld scopes.