I think it's a matter of cost and complexity vs. need.
Besides, you cannot get a good reading while underway in most situations because the fuel is moving in the tanks.
If I take a dipstick measurement in the morning and have 40 gallons (combined), I can estimate 20 hours of running 7 knots. Of course I wouldn't try that, I would leave a certain amount in reserve and also know that there is a certain amount that's below the pickup tubes.
Having a better estimate of fuel onboard could cut down unnecessary fuels stops. I'm willing to pay $5 for a dipstick but not $500 for a sophisticated gauge.