I'll answer only the "why not" question because I don't know about the feasibility of their plans.
Why nobody else seems to be trying this?
Because of two reasons.
Firstly, if you extract natural gas from a field, you can do the steam reforming over the ground. Then you will also have a place where to store the resulting carbon dioxide -- into a depleted natural gas field. So, if you really really want to do steam reforming with CO2 sequestration, there's no need to do it underground.
However, today in most regions that are not very far from the equator, it's already today cheapest to produce energy by solar power. The second cheapest energy production method is wind power. (Far away from the equator the order is reversed.) The trouble is, those have zero fuel costs. Meaning that when the amount of energy that can be produced exceeds the demand for energy, the price of electricity falls to zero. This doesn't make solar and wind power infeasible because they make most of their revenue when energy is scarce but not so scarce that production is zero. When there's absolutely no sunshine and wind (and energy is expensive), revenue is zero due to no energy available, and when there's plenty of sunshine and wind (and energy is zero-priced), revenue is zero due to zero-priced energy.
It is inevitable that the amount of installed solar and wind energy will become so large that we will see a very large fraction of all hours being these zero-priced hours (perhaps even free electricity for half of the hours). Thus, it makes far more sense to create hydrogen using electrolysis of water, because there aren't exactly that many ways free electricity could be put to useful use.
Today electrolysis cells cost around 1000 EUR per kilowatt, but because the technology is not rocket science unlike solar photovoltaics (that by the way cost today only 200 EUR per kilowatt), we will very likely see in the near future electrolysis cells at a price of 200 EUR per kilowatt.
Also, when the solar and wind share of all energy grows, the electricity consumed by these cheap electrolysis cells is zero-priced. You just turn the cells off when energy becomes too expensive.
Can't compete with that with any steam reforming process, whether it is happening over or under the ground, whether it is sequestering the CO2 or not.
And in the meantime, before electricity becomes free half of the time and before electrolysis cells reach a low price, the hydrogen can be produced over the ground with steam reforming, deciding whether to capture the CO2 or not based on the price of CO2 in the emission trading schemes.