Are they? A CubeSat can be launched for less than $100k, and their payload can contain off-the-shelf consumer hardware. That's what some upper-middle class families pay for an extra car. I think you're overestimating the cost of satellites.
At the same time, you're underestimating the challenges of operating a ground sensor network. How would identify which sites to survey from the ground without a broader search? Would the oil industry get to regulate itself? What about temporal variability or detecting flares? How would you detect new leaks and add new sensors? What about large-scale natural leaks that spread over many kms in inaccessible terrain?
Even assuming you could pinpoint the location of every active methane source ahead of time, how would you get a sensor there? How would you secure access for installation and maintenance? How would you ensure connectivity? How would you monitor in areas controlled by hostile governments?
Each of the hundreds of thousands of sites has its own logistical challenges which explode the cost. And because you need to keep the sensor network up to date, it's a constant operational battle, not a one time expense. Compare that to a satellite: the one-shot launch, then lower cost, lower operations, and superior temporal detection abilities and global coverage - not hard to understand why satellites are preferred.