Why no "off the sensor" metering?

Something I've been wondering is why they don't meter off the sensor in real time? My old olympus OM2n had the Off the Film metering (was A priority only), which was a light sensor facing backwards towards the film that measured the amount of light hitting the film and controlled the shutter in real time to get the correct exposure. It was VERY accurate even for very long exposures and if the lighting changed during the exposure, it would adjust for this.

The camera didn't need to know what the lens f stop was set at other than to give an estimate of the shutter speed via metering in the prism. The actual shutter speed was calculated during exposure.  So you could use lenses mounted in reverse etc and still use "A" mode. Also light going backwards through the prism had no effect on the exposure. It worked great with the dedicated flash as it was controlled by this same sensor. Macro lenses on a bellows and things like this were simple to use. I really loved that camera and miss how versatile and accurate that metering system was.

It seems like given the sensor itself is collecting light data, it would be possible to have the sensor itself tell the camera "OK, we have enough exposure" and close the shutter? Anyway just something I was wondering, seems like doing all the metering before the exposure is going backwards from some technology that was used 30 years ago.

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Stacey