It's "feasible" to use dynamic current sources per pixel, and many LED pixel drivers do offer this for dot correction / colour balance / global brightness, but

It's "feasible" to use dynamic current sources per pixel, and many LED pixel drivers do offer this for dot correction / colour balance / global brightness, but PWM is almost always used for pixel values; it's much easier to achieve the necessary resolution staying in the digital domain, and there's not really any downside. The other big issue with current control is that the simple ways to do dynamic current are linear, so effectively use constant power regardless of pixel state, and also burn a lot of silicon area and might start creating thermal issues in the driver. At high power levels, current regulated switch mode DC-DC drivers start to make sense, but doing that per pixel is definitely not feasible.