Why do oscilloscopes have such high sampling rates when the bandwidth is low?
The aim of this answer is to provide a basic representation of what an oscilloscope might display and, in order to make the explanation as accessible as possible (and cover the issues of sampling and aliasing), simplifications are used. A decent digital oscilloscope would use a reconstruction filter to make the displayed waveform better than what the raw output from an ADC would look like but, in my opinion, going into details about reconstruction filters is beyond the scope of the question.

why do we need such a huge sample rate?

1 Hz sinewave sampled at 10.4 Hz

It's not perfect - there are a few anomalous looking artefacts but, it's not bad (good enough for an oscilloscope).
1 Hz sinewave sampled at 2.35 Hz

Looking at the 2nd image, although it is still sampled above the nyquist rate, do you really want &quot;strange artefacts&quot; on your screen that might make you think that this is what your real signal looks like? Pictures from here.

Since we can capture adequately signals of frequency 14MHz (1/5 of
bandwidth rule of thumb,)

Sampling has to be greater than twice the maximum bandwidth of anti-alias filters in order to prevent aliasing artefacts making the images untrustworthy such as in this example of aliasing: -

So your rule of thumb doesn't make much sense because clearly, the final image directly above does not represent the original signal.