Ground-based sensors historically undercount methane emissions in 3rd party trials. They have a limited FOV which makes it hard to accurately measure a plume of gas, particularly when the wind blows. I suppose you could create a Dyson sphere of fixed sensors around an O&G asset, but that quickly becomes more expensive than a space based sensor.

Because these leaks can occur spontaneously, you really want real-time, continuous monitoring of a wide area with high resolution. Aerial sensors are impractical for this purpose. Only a large constellation of satellites can offer this.