Why are oscilloscope input impedances so low?
A lot of things are the way they are because of history, and de facto standardisation.

A general purpose oscilloscope input is a difficult compromise between not loading the circuit, not being damaged by high voltage, having reasonably low noise, and being able to maintain a decent bandwidth.

1Mohm in parallel with 15pF to 30pF satisfies a lot of people for a lot of applications. There's little incentive for manufacturers to build a general purpose oscilloscope with a different input, to address tiny parts of the market. 

When you do need better noise, or a differential input, or a higher input impedance, then you use a custom pre-amp. When you need wider bandwidth, you switch to a 50 ohm input impedance. 

There are special purpose oscilloscopes made at high prices that do address niche applications.